Method.—The following results on the histology of the sensory clubs, their eyes, and the tentacles, as already noted, were obtained from some of Dr. Conant’s preserved material. These results relate almost wholly to Charybdea, with only a few references to Tripedalia, noted in their proper place.

A portion of this material was killed after keeping the animals in the dark for some time, for the purpose of discovering any changes in the pigment of the eyes. I believe that a retraction of the pigment of the long pigment cells that project between the prisms and pyramids of the vitreous body in the retina of the distal complex eye is very evident in eyes killed in the dark. (But more on this below.)

I obtained my best results from the material preserved in saturated corrosive sublimate, to which had been added (5 to 10 per cent.) acetic acid. This also was Conant’s experience in his previous work on Charybdea and Tripedalia.

My best sections were obtained by embedding the sensory clubs in celoidin, passing the little blocks of celoidin with the sensory clubs into chloroform until perfectly transparent, and then into paraffine. I then cut sections as we ordinarily cut paraffine sections, mounted and stained them on the slide. My purpose in using this method was to avoid the displacement of the vitreous bodies of the eyes during embedding and cutting. This object was fully realized and more besides. Since the sections cut by the celoidin-paraffine method gave me so decidedly the best differentiation of the axial fibers of the retinal cells, as also of the cilia, basal bodies, etc., I am inclined to believe that the celoidin was in part responsible for this differentiation.

Most of my series were cut 4 µ in thickness. All in all I cut sixty-five clubs besides making some maceration preparations from material preserved for that purpose. These sixty-five series represent material from fourteen bottles. As a whole, my material was good, but the material from one bottle was decidedly superior for showing the axial fibers of the prisms and pyramids of the retinal cells. This shows the advantage of plenty of material. It will be evident that I had plenty of material.

I found iron-hÆmatoxylin the most satisfactory stain. I stained for a shorter or a longer time—one-half to several hours and longer—and then washed out the sections until under a low power of magnification they appeared quite unstained, the nuclei and a few other parts only appearing darkly stained.

Depigmentation I practiced but little. I obtained many of my series almost wholly unpigmented, especially those I cut last. Others, of course, were very heavily pigmented. I am not certain but that alcohol slowly dissolves out the pigment after a long period of preservation. Slight variations in the technique of killing and preserving may also, perhaps, determine the stability or solubility of the pigment, as, of course, also the condition of the pigment at the time of killing.

Anatomy.—For a short epitome of the anatomy of a Cubomedusa and of a Cubomedusan sensory club see p. 2 of the Introduction.

The Distal Complex Eye—General.—The distal (larger) complex eye (Fig. 7) and the proximal (smaller) complex eye (Fig. 13) are so named to distinguish them from the lateral simple eyes of the clubs. The distal complex eye consists of the following parts: a cellular cornea, continuous with the epithelium of the sensory club; a cellular lens (externally cellular and internally often quite homogeneous) immediately beneath the cornea; a homogeneous capsule just internal from the lens, and evidently a secretion from the lens cells; a vitreous body composed primarily of prisms and pyramids just beneath the capsule; and a retina of pigmented cells, with subretinal nerve tissue, ganglion cells and fibers. To my knowledge all observers (except CarriÈre, who missed the capsule) are quite agreed on the anatomical structure of the distal complex eye as also on the proximal complex eye and the lateral simple eyes.[d] It is on the histological structure of some of the various parts that differences exist.

Cornea.—Little need be said on the cornea except that it consists of flattened cells applied to the outer surface of the lens. It is continuous with the epithelium of the club and evidently a modified portion of this epithelium (Fig. 7). All observers conform to this statement.

The Lens.—The lens is of cellular origin, but in its interior the cells are often so changed—absence of nuclei, cell walls, and protoplasmic structure—as to make a mass quite homogeneous and structureless. While this internal mass sometimes shows practically no structure, yet at other times it is found broken up into masses much the size and shape of cells but without nuclei, while again, cells with nuclei may be quite evident. This occasional breaking up of this mass is evidently predetermined by its original cell structure. Iron-hÆmatoxylin stains this inner mass very dark and it is difficult to wash out the stain. Borax carmine and Lyons blue give the best results on the lenses. In figure 7 the lens of the distal complex eye is shown as quite homogeneous internally, while in figure 13 (proximal complex eye) it is drawn cellular. In this latter lens the inner cells are quite round and nucleated as they may also appear in the distal eye. What I have said applies equally to the lenses of both complex eyes, though the cellular nature of the inside of the lens is more readily demonstrated in the proximal eye.

It appears that it is in younger specimens that the central mass of the lens shows the cellular structure best, and that as the animal grows older this structure is more and more lost until no trace of it remains. As concerns most of my series I could not well determine which were from younger and which from older individuals, yet, several series of quite small (5 mm.) and therefore young animals, in which the eyes were so small that the lenses were compassed into less than half a dozen sections, the cellular structure of the lens was very evident.

The external cells of the lens form a spherical shell (both complex eyes) which, in section, shows as a hollow ring (Figs. 7, 13). The thicker ends of these cells lie at the inner (toward the capsule) half of the sphere and the cells taper toward the corneal surface, dovetailing laterally with their immediate neighbors as also distally with those from the opposite side of the sphere. The thicker inner ends of the cells contain the large nuclei with nucleoli. At a point (* Figs. 7 and 13) on the inner (next the capsule) surface of the lens the cells only approximate each other and thus leave a place which is easily broken through, as is shown by portions (drops, probably representing cells or portions of cells) of the mass within the lens becoming squeezed out into the substance of the capsule and the vitreous body, and found occasionally also among the cells of the retina. A considerable portion of the inside of the lens may be found thus squeezed out, and its path can often be traced. This phenomenon is evidently brought about by a contraction of the shell of the lens during fixation and before the inside of the lens has become hardened.

In origin the lens is evidently ectodermal, originating from an ectodermal invagination which becomes pinched off as a hollow sphere, the outer (i. e. next the cornea) half of which becomes the lens, the inner half the retina (i. e. vitreous body plus the so called retina). (See Retina.) The transition from retinal to lens cells is quite readily made out at the lower side of Fig. 7, but the corresponding structure on the upper left side is not so manifest. It is further evident that the lens is again an invagination into this sphere, and the point at which the lens cells approximate (where the central mass of the lens may be squeezed out as above described) represents the place of pinching off of the original lens-retina sphere. It appears, then, that the lens is formed in the lens-retina sphere in the following manner: The cells of the secondary invagination going to form the lens begin to lengthen distally (i. e. toward the cornea) during their invagination to form a hollow sphere, at the same time dovetailing with each other and budding off cells to form the inside of the lens (Figs. 7, 13).

At the lower side of the lens, near the margin of the retina, the cells of the lens are slightly indented or pushed inwards (Fig. 7, ind.). I believe this to be due to the weight of the lens in the normal position of the club, when the lens rests against the margin of the retina and the capsule and adjacent tissue.

Anticipating the description of the retina, it may here be added, that the retina is formed from the inner half of the lens-retina sphere. The cells of this portion of the sphere become differentiated into prism cells, pyramid cells, and long pigment cells, while laterally, beyond the margin of the vitreous body, they are differentiated into pigmented iris cells (Figs. 7, 6a).

Above are my results on the lens. Haake[2] speaks of the lens as consisting of a cellular “Kern” with a covering of lamellated cells. CarriÈre describes it as cellular and filled internally with a “Gerinsel,” or coagulation. CarriÈre and Haake are each in part right. Claus describes it as wholly cellular. Schewiakoff regards the lens as wholly cellular, and like Claus has not noted that internally this cell structure may be quite obliterated. Schewiakoff regards the lens and retina as formed from an invaginated sphere, and shows the transition from the lens cells into retinal cells as I have figured. Conant also gives the structure of the lens for the complex eyes as cellular but missed the change of structure that the interior of the lens may undergo.

The Capsule.—The capsule of the lens (Figs. 4, 7) lies immediately below (inward from) the lens. In structure it is homogeneous, except for certain fibers from the long pigment cells of the retina that traverse it, while sometimes also other fibers can be seen which, possibly, are branches from the fibers just mentioned or continuations from the fine fibers of the prism cells of the retina soon to be described. I have, however, no evidence that the fibers from the prism cells extend beyond the prisms in whose axis they lie. The capsule lies very closely applied to the lens, never becoming separated from it in sections, and is, hence, regarded as a secretion from the lens cells. Just what its function may be is difficult to surmise. The proximal complex eye possesses no capsule. I have thought, however, that if the lens should be adjustable, the capsule might serve as a protection to the prisms of the vitreous portion of the retina during the adjusting movements of the lens. (But more on this below.) To my knowledge all previous observers are quite agreed on the structure of the capsule. CarriÈre and Haake, however, missed it altogether.

Retina.—While I have enumerated (following previous observers) the vitreous body and the so-called retina as distinct parts, yet, as the sequel will show, they are, histologically, different parts of the same thing—namely the sensorium proper of the eye—and I propose to use the term retina for both taken together, while I retain the expression vitreous body (as hitherto used) for the vitreous portion of the retina. This simplifies matters; and using a word that is already used for analogous structures of other eyes (vertebrates, anthropods, molluscs) is conducive to clearness. I have been tempted, furthermore, to use the words rods and cones for the prisms and pyramids that I find in the vitreous bodies of the retinas of the complex eyes. But since the prisms in reality approximate prisms and the pyramids pyramids, in their shape, I have decided to retain the words prism and pyramid for these structures. The former of these terms (prism) was first used by Conant in his description of the complex eyes.

What I shall call the retina, then, in the distal and proximal complex eyes of Charybdea, consists of three kinds of elements: the prism cells, the pyramid cells, and the long pigment cells. (Figs. 4, 7, 22, prc, pyrc, lp.) We may also describe the retina as composed of three zones: the vitreous zone (vitreous body of authors), the pigmented zone, and the nuclear zone. (Figs. 4, 7, 22, vb, pz, nz.)

The cells composing the retina form a single layer in the shape of a hollow cup, into which cup the lens with its capsule fits. (Fig. 7.) This single layer of cells takes in the thickness of the vitreous zone, the pigmented zone, and the nuclear zone. Indeed, the distinctions vitreous zone (vitreous body), pigmented zone, and nuclear zone characterize three topographical regions of the retinal cells.

That the retina is made up of three kinds of cells is most readily demonstrated in transverse sections through the vitreous body. Fig. 1 is such a section, taken quite near the pigmented zone (at about the level x, Fig. 4). Three different kinds of areas are readily made out in such a section. The more numerous areas (pr) are transverse sections of the distal prisms of the prism cells, the less numerous and lighter areas (pyr) are transverse sections of the pyramids of the pyramid cells, and the large oval heavily pigmented areas (lp) are the transverse sections of the long pigment cells. The dots within the two first named areas represent fine fibers in the axes of the prism and pyramid cells, to be described below. The presence of three kinds of cells can again be readily seen in such Figs. as 4 and 7, in which the elements of the retina are cut parallel to their long axis. (Fig. 22.) Again, a transverse section through the most distal part of the pigmented zone of a slightly pigmented retina (Fig. 2) also shows us the presence of three kinds of elements. The larger and more heavily pigmented areas (lp) are the long pigment cells; the smaller, lighter areas (pyrc) with a central dot are the pyramid cells, and the more numerous dots, with no definite polygonal areas outlined about them (prc), belong to the prism cells. Thus, I believe, we have conclusive evidence of the existence of three kinds of cells in the retina of the distal complex eye.

(a) The prism cells are the more numerous, and, as the name implies, end distally in a vitreous polygonal prism (Figs. 4, 7, 22, pr). The prismatic structure of the vitreous body is also shown in Figs. 10 and 11, which are drawn from a macerated preparation of Conant’s. (See the descriptions of these figures.)

In Figs. 4 and 7 the prism cells correspond to the cells with the darker nuclei (npr); in Fig. 2 they are represented by the dots without defined polygonal areas about them (prc), and in Fig. 1 by the most numerous areas (pr). These cells, then, consist of a centrad portion with nucleus, a pigmented portion with granules of a dark-brown pigment, distal from the nucleus, and a distal vitreous prism which extends to the capsule of the lens.

In the axis of each prism is a fine darkly-staining fibril extending the entire length of the prism. I found no good evidence that this fiber extends into the capsule. Centrad this fiber is continued through the pigmented part of its cell and approaches to or near the nucleus (Fig. 2, dots without defined polygonal areas; Fig. 7, part of retina left unpigmented). In some instances I could trace this fiber quite to the nucleus, while in others it ended before reaching the nucleus or a little to one side of it. I am inclined to believe, however, that it extends past the nucleus and is continued as a nerve fiber. I believe this to be so because the fiber is evidently sensory, and a priori we should expect it to be so continued. Further, I find decided evidence in sections of the simple eyes to show that the fibers there extend past the nucleus into the subretinal tissue where I could not trace them farther. (Fig. 16.) Again, that the flagella of the epithelial cells of the club are also continued into the cells, in some instances could be traced past the nuclei (Figs. 12 and 26), and the fact, too, that the retinal cups of the eyes represent invaginated epithelium (the axial fibers of the prisms are hence cilia?)—all this leads me to believe that the axial fibers of the prism-cells extend centrad past the nuclei through their cells and are continued as nerve-fibers. (See below under pyramid-cells and under epithelium). Immediately upon entering the pigmented part of its cell the axial fiber of a prism-cell has a dumbbell-shaped enlargement which lies quite at the distal edge of the pigmented part of the cell (Fig. 7, unpigmented part of figure). This, of course, can be seen only in unpigmented retinas. This dumbbell-shaped body, (BasalkÖrperchen of Apathy), which name I give it, since it evidently is homologous to the basal bodies described by others for the cilia of epithelia, can be most beautifully seen as two minute spheres lying close together and in line with the nucleus. These two little spheres of the basal bodies put to the test the highest powers of the microscope; but, when, after a prolonged and careful study, one satisfies himself of their existence and exact shape, the very difficulty with which they are resolved adds a zest to be appreciated. The length of a basal body is about one-fifth to one-fourth that of the nuclei of the prism-cells.

The structure of the nuclei of the prism-cells is that of a dense network (Figs. 4, 7, npr) which stains dark with hÆmatoxylin. A nucleolus can often be seen in these nuclei. In some few series, again, these nuclei did not show a network-like structure, but the chromatin was arranged in masses (Fig. 5, npr). These nuclei can usually be distinguished from those of the other cells of the retina by their denser, darker-staining network (Figs. 4, 7, npr), or as shown in Fig. 5 (npr). Their denser structure and staining capacity are a distinguishing characteristic of the nuclei of the prism-cells. I must add, however, that not in every series is this apparent.

That portion of a prism-cell that contains the nucleus rarely contains any pigment; and when pigment is present, I believe that it has been dissolved in from the pigmented zone. The nucleus, again, lies a little centrad from the pigmented part of its cell, so that an unpigmented zone is seen in the retina between the pigmented zone and the row of nuclei (Figs. 4, 7, 22).

Centrad the prism-cells are continued as a single process (Figs. 6, b, c, d, and 8a, b, c, d). In some sections I thought I could trace these processes to the basement membrane, but I could not satisfy myself that such appearances were not due to artificial splitting in the tissue. Schewiakoff makes a similar remark about his supporting cells, which cells I believe are the same as my long pigment cells, but these do not extend to the supporting lamella.

At the margin of the retina the cells do not develop prisms but remain pigmented and form an iris (Fig. 7), which was so named by Claus and also described by Schewiakoff. These cells also assume a somewhat different shape (Fig. 6a). This cell (Fig. 6a) is seen from its broader side with which it is applied to the capsule or the lens. Schewiakoff figures similar cells. That the cells of the iris are prism cells without the prisms does not necessarily follow. They simply represent cells of the retinal cup that have become differentiated to serve as an iris.

As to the exact origin of the prisms, and pyramids (to be described below), it is difficult to say anything definite. If the so-called basal bodies of the axial fibers are really homologous with the basal bodies of flagella, then it would seem that they (the prisms and pyramids) are secretions comparable to cuticular secretions.

(b) The pyramid-cells, like the prism-cells, are differentiated into three regions: a distal vitreous pyramid, a pigmented part, and a centrad part with nucleus. The pyramids are seen in transverse section in Fig. 1 (pyr) and in longitudinal section in Figs. 4 and 7 (pyr).[e]

Each pyramid extends between the bases of the prism-cells about one-third to one-half the depth of the vitreous body (Figs. 4, 7, 12 (pyr)). The pyramids are also a shade lighter than the prisms, which fact is characteristic. In the axis of each pyramid is a darkly-staining fiber quite like the one described for the prism-cells (Figs. 1, 4, 7, 22). That this fiber extends distally beyond the limits of the pyramids I could not determine, but I do not think that it does. Centrad this fiber extends into the pigmented portion of its cell quite to or near the nucleus as was described for the fibers of the prism-cells (Figs. 7, 22). Whether or not these fibers extend past the nucleus and become continued as nerve fibers, the same course of reasoning holds as was given for the fibers of the prism-cells. Each of these fibers possesses a basal body just on its entrance into the pigmented part of the cell (Fig. 7), but I could not determine that it was dumbbell-shape. In form it represents an enlargement of the fiber itself, which gradually tapers again to its normal size. The continuations of these fibers within the pigmented parts of the pyramid-cells, as also the basal bodies, could only be demonstrated in unpigmented series.

Patten[5] describes axial fibers extending centrad through the rods (vitreous portions) of retinal cells (“retinophora”) into the region of the nucleus and past the nucleus (arthropods and molluscs). My retinal cells (prism and pyramid cells) evidently correspond to Patten’s retinophora, but I find no evidence that one of my retinal cells represents more than a single cell, while Patten gives evidence that his retinophora are made up of two cells closely applied to each other as twin cells. If this were also true for the retinal cells that I have described, I believe my macerated preparations would have shown it. Schreiner[12b] and Hesse[13] also figure and describe axial fibers for the rods of the visual cells in polychÆtous annelids, and Schreiner[12a] also for molluscs. Neither of these observers finds the fibers to extend distally beyond the rods nor centrad toward the nucleus as Patten and myself show. Neither Schreiner nor Hesse figures these cells as twin cells as Patten does, so that to my knowing Patten stands alone in this respect. Andrews[14] describes and figures rods for the visual cells of polychÆte annelids but no axial fibers. He was the first to describe these rods in annelids.

The pigmented zone of the pyramid cells, in heavily pigmented series, is filled throughout with dark-brown pigment granules, and is quite like that of the prism cells (Figs. 4, 7). In transverse sections, however, through the most distal part of the pigmented zone, of unpigmented series (Fig. 2), lighter areas with central dots could occasionally be demonstrated, which areas are the pyramid cells. In Fig. 2, the more definite polygonal outline as well as the lighter shade of these areas was a distinguishing feature. The difference in shade was not wholly due to a difference in pigmentation but to a structural difference.

The nuclei of these cells are usually a little larger than those of the prism cells and are filled with a finer and less dense network (Figs. 4 and 7, npyr), in consequence of which they present a lighter appearance in sections when examined with a high power. It will be seen in the figures (4, 7) with what regularity these lighter nuclei lie opposite the pyramids. Some few exceptions occur. These are probably due to the fact that a nucleus or pyramid was not differentiated by the technique. If this opposition between the pyramids and the lighter nuclei were all, I believe it would be sufficient evidence for associating these lighter nuclei with the pyramid cells.[f]

(c) The long pigment cells are about as numerous as the pyramid cells. In these cells, as in the prism and pyramid cells, three regions can be distinguished: the region of the nucleus, a pigmented region (the distal half of which extends between elements of the vitreous body), and a distal rod-like portion, or fiber, which is continued between the prisms into the capsule of the lens (Figs. 4, 7, 9). The pigmented portion is about twice the length of that described for the other cells, and also often of greater diameter, so that in transverse sections (Figs. 1, 2, 3) these cell-areas are larger than those of the other cells. As nearly as I could determine, these cells are pigmented just like the other retinal cells described. In quite unpigmented series, however, they often contain more pigment than the other cells do (Fig. 2). Distally, the pigmented part becomes narrowed to a strong pigmentless fiber (Figs. 3, 4, 7). This fiber stains quite dark with iron-hÆmatoxylin and appears homogeneous. It passes between the prisms into the capsule, where it usually bends in a direction toward the margin of the capsule (Fig. 7) and passes diagonally across this to the lens. In sections, a space is often seen about these fibers in the vitreous body, which I regard as a shrinkage space (Figs. 3, 4), since it is not evident in all series (Fig. 1). In Fig. 7, I have assumed that these spaces are due to shrinkage and have not indicated them. Also, in this same figure I have assumed that the spiral appearance of the fibers (Fig. 4) is due to a shortening of the prisms during fixation, and have drawn them straight. At the lens these fibers seem to end. In a few instances they were seen to branch upon reaching the capsule (Fig. 4). In Fig. 9, also, which shows some of these cells from a macerated preparation by Conant, the rods show evidence of branching at their distal terminations. In the same preparation I thought I could see that a fiber became expanded into a membrane spreading over one of the lens-cells. I could not satisfy myself, however, that this was the actual condition of things. Judging from Fig. 9, one might conclude that all the fibers are branched distally; yet, if such were the case I should have seen more of it in sections, but branching as seen in Fig. 4 is the exception. Hence, if all these fibers do branch, I am inclined to believe that it must be among the bases of the lens-cells. Or, if the fibers do expand into membranes to cover the lens-cells (I could not explain purpose), the evidence in Fig. 9 may be nothing more than fragments of this membrane left attached to the ends of the fibers. As is seen in Fig. 7, most of these rods end opposite the cells of the lens, and not usually between two adjacent cells as Schewiakoff has described for Charybdea marsupialis. The nuclei of these cells are like the nuclei of the pyramid cells (Figs. 4, 5, 7, 9) and often have a nucleolus.[g] Centrad these cells are continued into a number of processes as is seen in Figs. 5, 7 and 9. How far the several centrad processes extend and where they end I cannot say; but, as seen in Fig. 5, they soon taper to a thin end which I suppose may be continuous with a nerve fiber. I believe Schewiakoff was mistaken when he stated that these cells extend to the basement membrane.

I have found no evidence in these cells of the existence of an axial fiber such as I have described for the prism and pyramid cells. I find no definite arrangement of the nuclei of the retina into definite layers, but the nuclei of the three kinds of cells lie quite mixed, sometimes one kind lying deeper than the other as can be seen in the figures. Again, they may lie quite at the same level. (This point will be referred to later.)

It is these long pigment cells that I believe retract their pigmented part from between the prisms and pyramids when the medusÆ are placed in the dark, protruding with their pigment when placed in the light. Fig. 5 is a section from a slightly pigmented retina killed in the dark. The parts of the cells projecting beyond the pigmented zone, and which would lie between the prisms and pyramids (here not shown) of the vitreous body are seen to be narrower than in sections from retinas killed in the light (Figs. 1, 3, 4, 7) and the cells themselves appear in a condition of retraction as is shown by their large centrad portions with the nuclei, which latter, also, here lie at quite a lower level than the other nuclei. (The pyramid cells were not shown in this series.) I occasionally found appearances like Fig. 5 in retinas killed in the dark (indeed, in some the pigmented portions in the vitreous body were much thinner and more retracted than in Fig. 5). Yet this appearance was not of sufficiently general occurrence to leave no doubt as to its significance. As positive evidence, however, I cannot give it any other interpretation than the one given—that the cells retract themselves with their pigment when in the dark. Again, it must be added that the nuclei of these cells may occasionally lie quite deep even in retinas killed in the light. Indeed, like structures in different retinas may vary considerably in size and shape. None of my darkness retinas, however, showed such a large proportion of the pigmented parts of the long pigment cells projected between the prisms and pyramids as did the light retinas. I examined and tabulated all my series with respect to the extent the long pigment cells were projected into the vitreous body, and I found that those which showed these cells with their pigment least projected between the prisms and pyramids to be those that had been killed in the dark. I thus feel satisfied that the pigmented parts of these cells become in part or quite completely retracted from between the prisms and pyramids of the vitreous body when in the dark, but just how this is accomplished—whether the whole cell with its nucleus takes up a deeper position, the cell substance at the same time collecting in the region about the nucleus, as shown in Fig. 5 and the diagram (Fig. 22), I cannot with certainty state. It would seem, too, as though the pigment became less in the cells exposed to darkness, for I rarely, even in the most retracted heavily pigmented series, saw the pigment to extend farther towards the nucleus than commonly. The time of keeping in the dark, prior to fixing, varied from three-fourths of an hour to one and one-half hours. I could not bring the amount of retraction into relation with the time of exposure, except that in general the retinas longest exposed showed the greater amount of retraction.

(d) The tissue underlying the retina is described by former observers (Claus, Schewiakoff, Conant) as composed of nerve-fibers and ganglion cells. I cannot give it any other interpretation, but I must add that the supposed ganglion cells are seen only as nuclei, no cell bodies ever being demonstrable in any of my sections. Conant also recognized no cell bodies. Occasionally, as in Fig. 7, long fibers could be traced for some distance in this subretinal tissue, in some instances quite to or from a visual cell. Pigment was not regularly observed in this tissue, as Schewiakoff describes, and when present I believe it has been dissolved in from the pigmented zone.

(e) Schewiakoff describes the retina (my pigmented and nuclear regions) as composed of spindle-shaped visual cells (my pyramid cells?) alternating with pigmented supporting cells (long pigment cells), with the nuclei of the former lying more centrad than those of the latter. The visual cells are pigmented only at their periphery, or surface, leaving an unpigmented axis, while the supporting cells have pigment throughout their whole substance within the pigmented zone. Distally, the visual cells have hyaline rods, or fibers, which extend into spaces in the vitreous body, and pass through this and the capsule to the lens. The vitreous body is described as homogeneous, except the spaces for the visual rods, and a secretion from the retinal cells.

It will thus be seen that my results are quite different from those just described. I find the vitreous body to be composed of prisms and pyramids with axial fibers, while the long pigment cells (supporting cells of Schewiakoff) are continued into the vitreous body, and becoming narrowed into a non-pigmented fiber, extend to the lens as described. The prisms and pyramids are, further, the distal continuations of cells whose pigmented and nuclear parts lie in the so-called retina, but which, together with the vitreous body, I have named the retina proper. Conant has so summarily disposed of Schewiakoff’s distinction between retinal cells based on pigmentation and location of nuclei, that I need not say more. Schewiakoff’s Fig. 18 corresponds to my Fig. 1. In this figure he shows the vitreous body as homogeneous with pigmented areas (my long pigment cells) and with spaces with his visual rods. It is quite evident that his spaces with the visual rods correspond to my lighter areas with central dots; i. e. my pyramids of the vitreous body are the same as the spaces shown in his Fig. 18. It is quite evident that Schewiakoff mistook the lighter areas for spaces. That they are not spaces can readily be seen by comparing them with real spaces. It is, of course, possible, too, that the reagents had dissolved the pyramids, leaving only the axial fibers with a little pyramid substance about them, and that this is what Schewiakoff saw. I often found small circular spaces in the centers of the pyramid areas, as also in the prism areas (Fig. 3), which might be taken for hyaline visual rods, fibers, in transverse section, but in such spaces I could usually see a small dot to one side of the space that I take to be the rod (fiber) proper. Fig. 14 also shows such small circular spaces that have very much the semblance of hyaline rods. This figure is a transverse section of the vitreous body of the proximal complex eye, in which no long pigment cells or pyramid cells are present, but it serves well to illustrate the point. The above explanation also accounts for the large size of the visual rods (fibers) in Schewiakoff’s figures. That the fibers of the pyramid cells (visual rods of Schewiakoff) do not extend to the lens is quite evident in my Figs. 4 and 7.

Again, since the long pigment cells are often not seen to terminate in a fiber, but a part of the fiber can often be seen in the distal part of the vitreous body and in the capsule, it will be quite readily seen how Schewiakoff should associate his visual rods, or fibers, with these distal parts of the fibers of the long pigment cells and suppose his visual rods to extend to the lens.

Again, since the long pigment cells sometimes cannot be seen to terminate distally in a fiber, while the vitreous body at the same time may be broken away from the pigmented zone (Fig. 4), it is quite evident how Schewiakoff should have interpreted the parts of the long pigment cells in the vitreous body as conical pigmented caps placed opposite his supporting cells (long pigment cells).

Finally, since Schewiakoff had only twelve marginal bodies to study, and since this tissue is difficult to preserve properly, I do not believe that I am doing Schewiakoff any injustice by explaining away his results as I have done. This fact remains, that Conant and myself agree in all points in which we differ from Schewiakoff.

To Conant belongs the credit of having first demonstrated the prismatic structure of the vitreous body, and he also regarded the prisms as a part of the retinal cells. H. V. Wilson[15, 8b] suggested, however, some years prior to Conant, that the vitreous body might be of a prismatic structure. Conant had evidence also of both the prism and pyramid fibers, as is well shown in his figures of transverse sections but he found his evidence too meager to make any very definite statements. Indeed, Conant concludes that there are three kinds of fibers in the vitreous body and complains of finding but two kinds of cells in the so-called retina (pigmented and nuclear zones) to which to refer them. He saw the pyramids with their axial fibers as lighter areas in transverse sections of the vitreous body (his Figs. 64 and 68, and my Figs. 1, 4 and 7), but suggests that they may be the same as the long pigment cells, the cells having only to project themselves or their pigment in order to become long pigment cells. This suggested to him to preserve material both in the light and in the dark. I do not think Conant’s supposition to be a fact, for I find the pyramids in specimens preserved in the light as well as in the dark. It is, of course, possible that the pyramid cells are in a stage of structural transition to the long pigment cells, for, besides their pigmentation, they also have like nuclei. Furthermore, I held for a long time with Conant that there may be only two kinds of cells in the retina, but I soon found the pyramids so definitely shown as to leave no doubt but that they represented a third kind of cell. For me it remained to first definitely see all the fibers in the vitreous body as also the pyramids in sagittal sections.

Conant describes the long pigment cells with their fibers extending between the prisms of the vitreous body quite as I have described, and in this my work is only confirmatory of his. Conant does not, however, describe the several centrad processes of these cells, nor is he clear that their distad processes extend to the lens, though he speaks of fibers within the capsule.

(f) What, now, is the function of these three varieties of cells of the retina? Schewiakoff regards his visual cells (pyramid cells), as the name implies, as having a visual function. That they have such it seems reasonable to suppose, since they have an axial fiber in their pyramids. If the pyramid cells are visual cells, it appears that the prism cells also are such. Indeed, since these are the only ones present in the proximal eye and the more numerous ones in the distal eye, and like the pyramid cells have an axial fiber in their prisms, it seems that they are the visual cells par excellence of the Cubomedusan eye. Also, the analogy between the prisms and pyramids on the one hand, and the rods and cones of the vertebrate eye on the other hand, does not seem to be so far fetched. It may be of interest, here, to briefly consider Patten’s theory of color vision.[5b]

The gist of Patten’s theory is this: In the eyes of certain molluscs and arthropods, in the parts of the retinal cells corresponding to my prisms and pyramids, he not only finds an axial fiber (or fibers) but finer fibrils that extend at right angles from these axial fibers to the surface of the rods (I shall here, for convenience, call the prisms, pyramids, etc., rods) where they probably become continuous with other fibrils in the surface of the rods. These fibrils from the axial fibers are arranged in superimposed planes, and if I understand rightly, an axial fiber with its radiating fibrils may be compared to the axial wire with its radiating bristles of a brush used for cleaning bottles, provided the bristles of such a brush be arranged in superimposed planes. The lateral arrangement of the fibrils will, of course, be modified according whether a rod is circular, hexagonal, square, etc., in transverse section. It will also be remembered (p. 49) that Patten describes the retinal cells studied by him as composed of twin cells, and he gives the name retinophora to a pair. The system of fibers and fibrils in the rods he names a retinidium. Centrad the axial fibers are continued past the nucleus as a nerve fiber. The fibrils extending laterally in superimposed planes from the axial fiber of a rod, Patten supposes to be the ones stimulated by the incoming rays of light, the retinophora being so arranged that the light rays entering them are parallel to the axial fibers or perpendicular to the lateral fibrils of the retinidium. Again, since the rods are usually the shape of truncated pyramids or cones the lateral fibrils, which are perpendicular to the axial fibers, are of different lengths accordingly as they are situated at the larger or smaller end of a rod. Patten assumes similar fibrils to exist in the rods and cones (particularly the cones) of the vertebrate eye, and he thus makes a general application of his theory. He supports himself in this rather sweeping generalization by the claim to have demonstrated the twin-cell nature of the cones in amphibia and fishes.

For illustration, Patten supposes that if red light only were admitted to the retinophora this would stimulate the fibrils near the broader end of the cone (but that all the fibrils of the retinidium would be stimulated a little) and that we would thus have the sensation of red light. Likewise, if violet light only were admitted, the fibrils at the narrower end of the cone would be stimulated, and we should have violet light. Similarly, if light including all the different wave lengths of the spectrum were admitted, all the lateral fibrils would be stimulated and the sensation of white light produced. The method of stimulation need not be that of a vibration of the fibrils.

Certain grave objections may be raised against such a theory, the most serious, perhaps, being the fact that no such fibrils as Patten has described have as yet been demonstrated for the eyes of those animals that we know have color vision. Yet, as a whole, the objections are perhaps no more serious than any that can be brought against other theories of color vision. What Patten’s theory does do,—it gives us a definite mechanical basis to work from, and if these fibrils should be demonstrated for the rods and cones of vertebrates, physiologists would then have a mechanical basis for color vision quite as they now have for hearing. As Patten says, the problem is primarily a mechanical one. However, the theory cannot well pass for more than a suggestion, a stimulus for future work, and in this lies its present value.

It is quite evident that my results for the retinal cells of Charybdea are, if any thing, a support to Patten’s theory. While I have not been able to demonstrate the fibrils that are the essential to Patten’s theory, yet I have demonstrated the axial fibers of the rods, and if these fibers should be continued as a nerve fiber to some central ganglion (as I believe is reasonable to suppose, see p. 47), I do not see how we can avoid the conclusion that these axial fibers of the prism and pyramid cells are somehow concerned in vision. In Patten’s theory these fibers would represent a conducting element, the real sensory element (fibrils perpendicular to these axial fibers) not having been demonstrated by me.

I have recently read in a short review of Patten’s theory[9] that the evidence we at present have points to the tips of the cones (vertebrate eye) as being the seat of the sensation of red. This would be exactly the converse of what Patten’s theory supposes. Whether or not this objection is a real one, future investigation only can determine.

Hesse[13] regards the axial fibers that he describes for the rods in worms as the primitive fibers of Apathy. In this I agree with him, regarding the axial fibers I have described as “Primitivfibrillen.” Further, I believe, if I understand Apathy rightly, that the fibrils described by Patten as extending laterally from the axial fibers correspond to Apathy’s “Elementarfibrillen.”

It is the long pigment cells that are the puzzling element. Since there can be little doubt but that these cells can project and retract their pigmented parts (as already described), it would seem that a part of their function is to check the diffusion of light in the vitreous body when exposed to strong light. This function would be quite analogous to that of the pigmented cells of the vertebrate retina, which in light become projected between the rods and cones. Similar observations have also been made on the compound eyes of arthropods by Herrick[10] and by Parker[7], who find that the distal retinula cells of PalÆmonites project themselves distad in the dark, thus surrounding the vitreous cones with a cylinder of pigment, while (Parker) the pigment of the proximal retinula cells migrates centrad and the accessory cells move distad; in light the reverse takes place. Other observations of this kind are not wanting for crustacea, insects and arachnids. To my knowledge, the pigment changes that I have described are the first of their kind for medusÆ.

I suggested while describing the capsule, that the lens might be adjustable. That the fibers of the long pigment cells extend to the lens is my principal reason for this. May these cells not represent ganglion cells and their distad fibers nerve fibers? That they are not sensory (i. e. are stimulated by light waves) seems to be suggested by their not having any axial fiber and in having several centrad processes. These facts suggest that they are not sensory but the center of a reflex mechanism.[h] When the sensory cells proper are stimulated, the impulses are conducted centrad into some nerve center (it may be the nerve tissue underlying the retina, or other nerve centers such as the two groups of ganglion cells in the upper part of the club, or the radial ganglia) from which center, again, impulses return over fibers leading to the long pigment cells causing them to project their pigment, and conducting the impulse to the lens, to produce a change in its adjustment. Since these cells are not so numerous as the prism and pyramid cells taken together, but in turn have a number of processes continued centrad (the sum of which processes approximates the number of sensory cells, prism and pyramid cells) it appears that these cells are admirably adapted to function in just such a mechanism as I have described,—each long pigment cell serving a number of its immediate neighbors.

Further, we may conceive each of the centrad processes of the long pigment cells as receiving a fiber from one of the sensory cells directly as well as indirectly, as just described. While I have been able to demonstrate only a single centrad process for the sensory cells (prism and pyramid cells), yet this does not exclude the possibility of a nerve fibril passing out from such a centrad process to one of the processes of the long pigment cells, and it seems possible that this constitutes the reflex mechanism. That nerve fibrils ramify in ganglion and sensory cells, and may even leave these cells to join those of other cells, has been well demonstrated by Apathy,[6] so that my finding only a single process of the visual cells leading centrad without giving off lateral fibers cannot be a serious objection. Again, fine nerve fibers coming off from the main centrad process of sensory cells in medusÆ have been figured by other observers, among whom I mention the Hertwigs. Careful macerations at the seashore would probably demonstrate them for Charybdea.

Hesse thinks that the eyes of the AlciopidÆ are adjustable. He describes what he supposes to be muscle fibers just exterior (distal) to the lens, and believes that a contraction of these fibers would have the effect of forcing the lens nearer the retina, or vice versa. His supposition, like mine, needs experimental verification. Hitherto the only instance known of accommodation in the eyes of invertebrates was that described by Beer[17] for Cephalopods.

The Proximal Complex Eye.—With four exceptions, the description and discussion given for the distal complex eye also holds good for the proximal complex eye (Fig. 13). The four exceptions are: the absence of a capsule to the lens; the absence of the long pigment cells; the absence of the pyramid cells; and the different relative position of the lens and retina. This eye, then, has a cornea continuous with the epithelium of the sensory club, a lens, in structure and probable origin quite like that described for the distal complex eye, and a retina of prism cells with axial fibers for the prisms. Since Conant[8b] has described this eye quite fully, and discussed Schewiakoff’s conclusions at length, I shall be brief. Suffice it to say, that Schewiakoff describes two kinds of cells (supporting cells and spindle-shaped visual cells) for the retina of this eye just as he described for the distal complex eye. The vitreous body he likewise describes as being homogeneous and with spaces for the visual rods (fibers) of the visual cells. It is evident that Schewiakoff has interpreted the structure of this eye from analogy with his results on the distal complex eye. Claus likewise has described two kinds of cells for the retina, and the vitreous body as homogeneous. Conant and myself find only one kind of cells in the retina of this eye. The pigmentation that Schewiakoff describes for the vitreous body I believe to have been dissolved in from the pigmented zone of the retina, for I find no regular pigmentation in the vitreous body. Haake’s observation, previously noted (p. 42), applies also to the proximal complex eye.

Conant’s evidence for the axial fibers of the prisms was clearly insufficient, so that he did not in this respect complete his Fig. 69. I republish this figure with the prism fibers drawn (Fig. 13).

Since the long pigment cells are absent my reasons for supposing the lens of this eye to be adjustable vanish.

Finally, a word on the origin of the lens and the relative position of the lens and retina. The lens and retina in this eye are evidently not developed from an outer and an inner half, respectively, of the invaginated and pinched-off lens-retina sphere (as is true for the distal complex eye) but from proximal and distal halves respectively. It is also quite easy to understand the connection of the lens in this eye with the supporting membrane. Since the cells of the ectoderm of the club can in many instances be seen to extend to the basement membrane, or supporting lamella, the cells of the lens, which arise from the ectoderm, simply remain in connection with the basement membrane, this becoming thickened to form a support for the lens. That the lens of the distal complex eye has lost its connection with the basement membrane is evidently due to the fact that the lens is formed from the outer half of the lens-retina sphere. The cells of the lens are by this so far separated from the basement membrane as to lose their connection with it. Schewiakoff also notes the fact that the lens and retina of the proximal complex eye are developed from proximal and distal halves of the lens-retina sphere. He further supposes that the portion of the basement membrane that acts as a support to the lens takes the place of the capsule in the distal complex eye. This latter supposition I do not think probable, since the supporting lamella does not form a distinct covering to the lens on its retinal side.

The Simple Eyes.—Since the shape and position of these eyes have already been described (Claus, Schewiakoff, Conant), I shall not tarry long in this respect. Speaking generally, these eyes are flask-shaped (Fig. 12), the proximal pair quite so, while the distal pair are drawn out in the transverse diameter of the club. These eyes are invaginations of the surface epithelium and the shape of the cells lining these invaginations is quite like that of the epithelial cells, except that their distal portions (bordering the lumen of the invagination) are heavily pigmented. The proximal walls (Fig. 12, left side) of the distal pair are heavier pigmented than the distal walls and the proximal pair of eyes. Schewiakoff calls attention to this point. The pigmentation is, furthermore, not only heavier, but the pigmented portion of each cell is much longer in the proximal walls of the distal eyes (indeed, the cells are longer) than in the distal walls. The significance of this I do not understand. Indeed, I am inclined to believe that in life all these eyes are pigmented quite alike and that it is the reagents used that alter or dissolve the pigment in certain places. Yet, the fact that the cells of the proximal walls of the distal eyes have their pigmented portions nearly double the usual length, shows some deeper significance.

I also note here the small secondary, non-pigmented invagination into the tissue of the clubs from each of the distal simple eyes. Schewiakoff describes this invagination, and it extends in a proximal and dorsal direction (dorsal-side of club opposite complex eye) from the dorsal sides of the distal simple eyes. The cells of these invaginations are not pigmented, but quite like the other pigmented cells in shape, and like these with distal flagellate fibers. I do not see the necessity of assuming, however, that these secondary invaginations are the real sensitive parts of these eyes, while the pigmented parts serve as an iris, as Schewiakoff does in his general discussion.

The histological structure of both pairs of simple eyes is the same. Sections and macerations give me evidence of only one kind of cells, all pigmented alike (except, of course, the non-pigmented secondary invaginations just noted). The cells in these eyes are very closely crowded so that their nuclei lie at several different levels. That they all extend to the lumen of the eyes and are all pigmented could be demonstrated with certainty in many sections, when some of these cells whose nuclei lay most centrad could be followed with the greatest nicety to the lumen (Fig. 12). Macerations (Figs. 8, unlettered cells 21) also show cells with very long cell bodies pigmented at their distal ends and occasionally with a distal process or fiber. While there are, therefore, spindle-shaped cells found, yet they are in every other respect alike, and their differences of shape and position of nuclei are simply the result of crowding. There is, therefore, no evidence of supporting (pigmented) cells and spindle-shaped visual cells (pigmented only externally) as Claus and Schewiakoff have described and which Conant and myself cannot corroborate.

Distally, the retinal cells of the simple eyes have each a fiber (flagellum) that extends into the lumen (Figs. 12, 15, 16, 21). Each flagellum has a dumbbell-shaped basal body just on its entrance into its cell quite like the basal bodies described for the visual cells of the complex eyes (Fig. 12, part left unpigmented). Each flagellum, or fiber, can usually be seen to extend into the cell. In one series I found appearances like Fig. 16, which is a drawing of a part of a section through one of the proximal simple eyes. This section is quite in the angle between the proximal complex eye and the group of network cells in the upper part of the club. In this series I could very definitely trace the distal fibers of the retinal cells centrad, past the nucleus and into the subretinal nerve-tissue. These fibers could be so easily followed that no doubt can exist as to the fact noted. It thus appears that the axial fibers just described pass centrad through the cells and are continued as nerve fibers. On the evidence of such sections as Fig. 16 I have indicated these fibers as extending centrad through their cells. The lumen of the simple eyes is filled with a homogeneous vitreous secretion. This is often incomplete in some parts; occasionally the secretion shows a formation of globules, but all this I believe to be due to the action of reagents. Indeed, I have found simple eyes in which hardly any secretion was present, while others showed an almost completely filled cavity. In that portion of the vitreous secretion just outside the mouth of the distal eyes I occasionally found numbers of very darkly staining granules. I suspect that these are either bacterial or algal organisms.

As already noted, Claus and Schewiakoff describe two kinds of cells for the retinas of these eyes which neither Conant nor myself can demonstrate. Further, I believe I have shown that only one kind exists. If any doubt should still exist, a section like Fig. 25 (which is from the epithelium of the club, but similar smaller areas with central dots could often be demonstrated in transverse sections of the retinal cells of the simple eyes) I believe should be convincing. Schewiakoff further describes flagella for the retinal cells (his visual cells) of the simple eyes quite as I have described them for all the cells. The pigmentation that Schewiakoff mentions as occurring in the secretions within the lumina of these eyes I believe to have been dissolved in from the pigmented zones. I find no definite pigmentation in these vitreous secretions. These secretions are evidently products of the retinal cells and have been so regarded by former observers.

Lithocyst and Concretion.—The cavity filled by the concretion is lined in places by a single layer of cells, two of which are shown in Fig. 7. This fact has been noted by both H. V. Wilson and Conant. Such cells are evidently remnants of the cells that formed the concretion. The supporting lamella completely surrounds the cavity of the concretion.

The concretion filling the lithocyst has the shape of a hemiprolate spheroid cut in the plane of the axis of revolution. Whether it is of endo- or of ectodermal origin, I believe developmental studies only can determine. Tests made in the Chemical Laboratory show the presence of calcium sulphate with perhaps a very small trace of phosphate.[i] Nitric acid slowly dissolves these concretions, but I believe Claus was mistaken when he said that they dissolve with an evolution of gas. I watched them dissolve under the microscope, and never could see the least bit of gas formed. If Claus’s observation is correct, then the composition of the concretions of C. marsupialis is different from that of the concretions of C. Xaymacana. The concretions, further, were dissolved out of the material preserved in formaline and in osmic acid solutions. For dissolving them in situ I used either nitric or hydrochloric acid, or both. A slight husk remains after all the lime is dissolved.

The Epithelium of the Clubs.—The epithelium is thickest on the dorsal side of a club. The thickening here, as in several other places, seems to be due to a crowding of the cells, in consequence of which the nuclei come to lie at different levels, but I believe that all the cells quite reach the surface. The cells with their nuclei nearest the surface are pyramidal in shape, with the bases of the pyramids toward the surface, while those cells whose nuclei lie deeper (where several layers of nuclei occur) may be spindle-shaped (Figs. 12, 23, 24, 26). Centrad these cells are continued into a single process, which often seems to extend to the basement membrane (Figs. 7, 12, 13, 23, 24). Where the epithelium covers the region of the concretion, the cells become flattened and with the long axis of their nuclei parallel with the surface of the club (Fig. 7). The same holds true for the corneal epithelium (Figs. 7, 13).

It is a significant fact that in many places the nuclei form only a single layer, and in such places one cannot speak of spindle-shaped cells. I cannot find any evidence of sensory and supporting cells as Schewiakoff describes. The fact that spindle-shaped cells may exist is simply a physical consequence of their being closely crowded. Conant arrived at the same conclusion.

But I have another and better reason for supposing the existence of only one kind of cells in the epithelium. In a tangential section taken just through the tips of the epithelial cells (Fig. 25) I find polygonal areas with a central dot. This section does not at all agree with Schewiakoff’s Fig. 8, in which he figures two kinds of cells. In Fig. 25 there can be no evidence of two kinds of cells, unless both kinds have like flagella, for these dots are the transverse sections of flagella continued within the cells (Fig. 26).

The epithelium, then, is flagellate, a flagellum to a cell. Whether there are flagella on the epithelium covering the region of the concretion, I could not determine. But I believe that in all other parts, excepting, of course, the corneas, it is flagellated. The fibers (flagella) of the simple eyes are evidently the flagella of the invaginated epithelium. Each flagellum has a basal body, and I could in many instances determine that it was dumbbell-shaped (Fig. 12). This fact was not always evident, however, and it was only occasionally that I felt sure of it. Often the flagella showed only a general thickening within the cells (Fig. 26) while, again, the thickening (basal body) might be quite localized near the surface of the cell. Each flagellum extends into its cell, and occasionally I could trace one clear past the nucleus into the subepithelial nerve-tissue (Fig. 26), just as I did for the axial fibers of the retinal cells of the simple eyes. In those instances in which I could do this, the fibers could so clearly be traced that little if any doubt can exist. I have thus made bold and have drawn the flagella as continued through their cells into the subepithelial nerve-tissue for all the cells of the epithelium of Fig. 12.

A word on the epithelium covering the network cells of Fig. 13. Conant and Schewiakoff here describe fibers from the supporting lamellÆ that pass in bundles in among the network cells. These fibers are supposed to be a part of the supporting lamella which reaches out to be a support for the epithelial cells. (Schewiakoff also describes similar fibers for other parts of the epithelium.) Now, as Conant himself shows in Fig. 13, these coarse fibers are not of the same consistency and staining capacity as the supporting lamella. I found them to stain just like the intracellular parts of the flagella or like the central continuations of the axial fibers of the cells of the simple eyes. I could, also, occasionally trace them to the surface of the epithelium, and beyond, when they became continued as short blunt processes or flagella (Fig. 13). I, therefore, conclude that they are sensory fibers like those I have described for the other epithelial cells. Yet, that they pass to the supporting lamella, just as Conant shows in Fig. 13, would seem to indicate that they are fibers from the supporting lamella or processes of the epithelial cells. While this stands as an objection to their being sensory fibers, yet I cannot explain away their being continued distally as a flagellum, except I assume this continuation to be an artefact. This does not seem probable. Perhaps they serve both purposes; namely, that the cell body with its axial fiber is continued to the supporting lamella, the cell proper ending there, while the axial fiber is continued as a nerve fiber. I believe this to be the proper explanation.

The epithelium of the peduncle is quite like the epithelium of the club just described. Sections through the tips of the epithelial cells of the peduncle and also sections sagittal to the axis of these cells give sections like Figs. 25 and 26. I, therefore, conclude that this epithelium is a sensory flagellate epithelium like that of the clubs. Nerve tissue and unstriped muscle fibers underly the epithelium of the peduncles. Claus and Conant also describe a small ventral endodermal tract of nerve tissue, which according to Conant is connected with the endodermal nerve tissue found in the region of the radial ganglia.

To sum up, the epithelium of the club and the peduncle is a flagellate sensory epithelium whose flagella are continued through the cells as nerve fibers into the nerve tissue below. A priori, judging from the mass of nerve tissue underlying the epithelium, we should expect the epithelium to be one strictly sensory. What sense it serves is difficult to surmise. In the physiological part of this paper I suggested that it might be tactile, serving in connection with the lithocysts in giving the animal sensations of space relations.

Claus mentions having seen patches of flagella on the epithelium of the clubs. Schewiakoff supposes that his spindle-shaped sensory cells have only a single flagellum, while his supporting cells have many cilia. In the latter supposition he was evidently mistaken. Conant (from an unpublished note) saw the flagella of the epithelium on the living object and does not think that there could be more than a single one to each cell. He also concludes from living specimens squeezed out under a cover-glass, that there is only one kind of cells in the ectoderm.

Cilia and flagella extending into the cells to which they are attached are described by a number of observers.

I shall not endeavor to discuss the subject further, but shall append the literature on the subject that has come to my notice. (See Literature). Some of these observers ascribe a nervous function to these centrad continuations. I am inclined to believe that they represent the primitive fibrils of Apathy, whether the cilia or flagella are motile or sensory. I should mention, however, that Apathy has traced the “Primitivfibrillen” to be continuous with cilia, and also traces them into the sensory rods of the sensory cells in the sense organs of leeches. Eimer also describes cilia as continued centrad.

The Network Cells and the Multipolar Ganglion Cells.—Conant is the first to accurately describe the true structure of the network cells (Fig. 13) that fill the upper part of the club between the proximal complex eye and the attachment of the peduncle. I cannot add anything to Conant’s description. As their name implies, they are filled with a coarse network-like structure with a central nucleus and nucleolus. Schewiakoff erroneously described them as ganglion cells and Claus as supporting cells. I have sometimes thought that they are not made up of a network, but of a vesicular structure; i. e. the network we see is really produced by the sections of planes that intersect to form little polyhedral cavities. I could not, however, satisfy myself on this point. I further saw similar but smaller cells, with a finer network, disposed in small groups laterally and distally from the attachment of the peduncle to the club.

What the function of these network cells is can only be guessed. In size and shape they somewhat resemble some of the cells found in luminous organs. Conant, however, nowhere mentions that Charybdea is luminous.

Lateral to the larger group of network cells lie two groups of large multipolar ganglion cells (a group on each side). Claus describes these cells, but Schewiakoff does not specially note them, and evidently considered them a part of the network cells, which he erroneously described as ganglion cells.

The Nerve Tissue.—I cannot add anything new on this. It consists of fine fibers and ganglion cells, quite as described by Claus, Schewiakoff, and Conant, and fills the club between the ampulla and the epithelium, except the spaces occupied by the eyes, lithocyst, and network cells. It is likewise present under the ectoderm of the peduncle, where also a small tract is found under the endoderm. (See preceding head, or Claus[3], and Conant[8b]). As already noted, under the distal complex eye, I find only large nuclei to represent the ganglion cells. By saying this, however, I do not wish to dispute their ganglionic nature. The large multipolar ganglion cells I have noted under the preceding topic.

The Supporting Lamella.—The supporting lamella is a continuation, through the peduncle, of the jelly of the bell. It completely surrounds the ampulla and the lithocyst, and also forms a partition between them, so that, as already noted, the lithocyst becomes completely surrounded by it. It also sends a partition ventrally between the complex eyes (Figs. 7, 13). Its thickening to form a support for the lens of the proximal complex eye has already been noticed. I shall limit myself in the discussion of the supporting lamella to the above short resumÉ, since Schewiakoff gives further detail.

The Endothelium of the Ampulla and the “Floating Cells.”—The ampulla is lined by a secreting epithelium. This is shown by the large masses of a secretion within the bases of the cells, and by smaller masses scattered in the central and more distal parts (Figs. 7, and 27, lower half). The section of the cells is such in Fig. 7, that the bases of some (those nearest the supporting lamella) are taken, the central nuclear region of others, and the tips of those farthest from the supporting lamella. The section may be said to be taken diagonally through the bases and central parts of some of the cells, but owing to the curvature of the ampulla wall, through the tips of others. The secretion is a colloid substance, staining yellowish gray with iron-hÆmatoxylin, blue with Lyons blue, and reddish with borax-carmine. Sometimes darkly staining rods and fibers of unknown origin could be seen within the larger masses of the secretion (Fig. 7). These rods and fibers could also be seen in spaces within the cells, from which the secretion had evidently been dissolved. I think there can be no question but that the masses described are a secretion. Many series, however, do not show it; indeed, an examination of Conant’s slides gave me little evidence of a secreting function, though I could demonstrate it in his sections both within the endothelium and also the floating bodies. The presence or absence of this secretion is evidently correlated with the feeding habits of the animals, or else it would be more generally present.

The endothelium is thickest (the cells are longest) in the upper part of the ampulla where the supporting lamella approaches the lens of the proximal complex eye, and in the lower portion of the ampulla (Fig. 7), in the angle between the concretion cavity and the region of the distal complex eye. In general, the cells are longest in the upper part of the ampulla, while in the lower part, especially where they cover the concretion cavity and the dorsal wall, they may be quite cubical instead of columnar. Often they present a vacuolated appearance at their bases (Fig. 27). Claus and Schewiakoff describe and figure this endothelium, but not in detail. No one, to my knowledge, has described this secretory function.

The nuclei of these cells are peculiar. They may contain a network with a nucleus (Fig. 27). Again, they may show evidence of amitotic division (Fig. 20, h, i, j). Indeed, Remak’s scheme (Wilson[18] “The Cell,” p. 46) can be quite readily demonstrated. It is, however, such dumbbell-shaped, elliptical, or ringed nuclei as seen in Figs. 7 and 20 that are of special interest.

I have spoken of some of these nuclei as dumbbell-shaped, elliptical, or ringed. This is so, however, only in sections. They are really flattened spheres with a rod of tissue, of the same structure as the nuclear wall, stretching between the poles. One may conveniently compare the shape of these nuclei with that of an apple, the core of the apple representing the rod connecting the two opposite flattened or slightly hollowed poles of the nucleus. For convenience I shall call the rod connecting the two poles the axis of the nucleus. The dumbbell or elliptical shape would be obtained by a meridional section through the axis (Figs. 20, a, b, c, e, g, k, l, m, n, o, 7). Likewise a ringed appearance with a central dot would be obtained by a section parallel with the flattened surfaces or perpendicular to the axis (Figs. 20, d, 7). In a section not strictly meridional the axis would be cut as in Fig. 29, a, or not show at all. As nearly as I could determine, the inside of these nuclei is a vacuole, which the axis penetrates.

The walls and axis of these nuclei have the structure of a very fine and dense network that stains very dark with iron-hÆmatoxylin. It stains quite like the reticulum of any nucleus, but is very dense, as though all the reticulum of the nucleus had been crowded together at the surface. Judging from appearances like p (Fig. 20), the hollowing out, so to speak, of these nuclei, would seem to be a process of vacuolation, the reticulum becoming crowded aside to the surface. But how, on this view, to amount for the formation of the axis, I do not know. Perhaps the axis is formed by a pushing in of two opposite poles of a nucleus, the two invaginations meeting and fusing. On this supposition one might expect the axis to be hollow (cylindrical), but I could not determine that it was. Perhaps the centrosphere (or spheres) (see the next paragraph) has something to do with the formation of the axis (Fig. 20, b, g, e, etc.).

In the nuclei of Fig. 20 with the dark outlines, and of Fig. 7 a small reticular body is seen just opposite one end of the axis, or opposite both ends in g. In d (Fig. 20) this body is seen next the axis just below (outside) the hollow cup represented by the hollow ring. In this instance a central granule is seen in the reticular body, as also in c. I take this reticular body to be the centrosphere, and the central granule in c and d the centrosome. In k, l, m, n, and o (Fig. 20), which are from another series, in which the walls of the nuclei did not stain so dark as in the other nuclei of the same figure, a nucleolus could be definitely seen, indeed, sometimes quite perched upon the wall of the nucleus (k, l). In several instances I could see two nuclei, as in o. But besides these nucleoli, I could in several instances see quite definitely a reticular body (centrosphere) opposite the axis (m, n, o) quite as I described for the nuclei with the dark outlines. In a, b, c, d, e and g the nuclei could not be so readily demonstrated, but I could occasionally see a darker stained body as in a, c and g, that I have no doubt is the nucleolus, which here, again, is perched quite upon the surface of the nucleus. This position of the nucleolus is perhaps due to its having been crowded to one side by the nucleus becoming hollow. It is no uncommon thing, either, to find several nuclei in a single cell, sometimes in process of division or just divided as o and e (Fig. 20), also h, i and j. The whole nuclear phenomenon that I have described seems to be one of division. Perhaps it is somehow associated with the giving off of the secretion of the cells, for these nuclei seem to be found in greatest abundance in those cells in which the secretion is most abundant. In Conant’s sections I found but little evidence of these nuclear phenomena as also little secretion, which all goes to show the association of the nuclear phenomenon with the secretion. I have failed to find any descriptions in the literature of nuclei to which I could refer my observations.

The endothelium of the ampulla is flagellated (Figs. 7, 17, 27). It will be seen that there are two slender flagella to a cell. Each pair of flagella has a pair of basal bodies that are longer than thick, and which are continued as a thin fiber towards the nucleus of the cell. That these centrad continuations of the basal bodies extend to or past the nucleus I could not determine. Sometimes the basal bodies with the centrad continuations are pushed quite to one side of the cell (Fig. 27), while in other cells they are applied quite to the distal surface (Figs. 7, 17, 27). Fig. 17, and the part of Fig. 7 that shows these points, are taken just through the tips of the cells. The darker lines within the polygonal areas are the intracellular basal bodies with their centrad continuations, while the thinner lines are the flagella, and are supposed to lie in the plane just below the plane of the figure. In those instances in which the centrad continuations are applied to the distal surface of the cells they could occasionally be seen to bend centrad (Fig. 27b). While these cilia with their basal bodies and centrad continuations are usually separate, as shown in the figures, yet they are at times applied quite closely to each other so that the double nature of the basal bodies and their centrad continuations is not evident. When the intracellular continuations of the cilia become pushed to one side or applied to the distal surface of the cells, I believe this to be due to the turgor of the cells consequent upon the deposition of large masses of secretion within them. But I must add that this explanation is not altogether satisfactory, since in the endoderm cells of the pedalia of both Charybdea and Tripedalia I found like conditions with no evidence of a secreting function. (See below, under tentacles.) No one, to my knowledge, has described the flagellation in detail, although both Claus and Schewiakoff state that the endoderm is ciliated.

The “floating cells” in the stomach pockets and in the ampulla, described by Conant, I believe are in part derived from the endothelial cells of the ampulla. That a portion of them may arise from the ovary, as Conant explains, I do not doubt; I have, further, found a mass of floating cells in a small Charybdea quite as Conant describes for Tripedalia (his Fig. 71). In this Charybdea, however, I could find no traces of any ovary. Conant speaks of larger and smaller floating cells, and that the smaller ones are also found in the males. This latter fact agrees with what I have suggested, that some of the floating cells arise in the ampulla. My chief reasons for my supposition, however, are the following: I find globules of the secretion of the ampulla cells in some of the floating cells and also scattered loosely among them (Fig. 19). These globules in and among the floating cells have the same general appearance and a similar staining capacity as the secretion in the ampulla cells. Again, in spaces within some of the ampulla cells I find bodies resembling the floating cells with lumps of the secretion within them (Fig. 18). The conclusion, therefore, lies near that some of the floating cells originate within the cells of the ampulla, engulf within them some of the secretion, and are then expelled into the lumen of the ampulla. Better said, perhaps, they represent portions of the ampulla cells with some of the secretion. I also found several instances in which a floating cell had the appearance of being expelled from an ampulla cell. Conant suggests for a similar observation that the cells were about to be swallowed by the ampulla cells. I believe, however, that my finding a secretion similar to that within the cells of the ampulla, in some of the floating cells, as also bodies very much like them and filled with secretion within the ampulla cells, together with Conant’s finding floating cells in males, and finally the observation that the floating cells are usually quite dilapidated, never showing a healthy cell structure—all this leads me to conclude that some of the floating cells originate from the ampulla cells, and that they have a nutrient function in distributing the secretion. This is quite the reverse of what Conant supposed,—that they were taken in as nourishment by the ampulla cells. I also find what appears to be a secretion in the endoderm of the tentacles of both Charybdea and Tripedalia, and believe this is another source of the floating cells. (See below, under tentacles.)

I also found other very darkly staining bodies (Fig. 19) both within the floating cells and free in the ampulla cavity, and more numerous in the ampulla cells themselves. This again goes to show that floating cells take their origin from the ampulla cells. What these darkly staining bodies are, I cannot say. Perhaps they are something akin to the “Chromatoider NebenkÖrper” described by Lenhossek (L), or they represent another kind of secretion. If these floating cells are derived from the cells of the ampulla, the active nuclear division within these also receives an explanation. Some nuclear matter can usually be observed in the floating cells.

The Endothelium of the Peduncle.—The endothelium of the peduncle consists of flagellate columnar cells (Fig. 27, upper half). The cells are vacuolated at their bases like some of the cells of the ampulla, and contain a comparatively large nucleus with nucleolus. The flagella are long and slender, quite like those described for the cells of the ampulla, except that there is only one to each cell. The basal bodies of the flagella are of a peculiar shape. They may be described as a bent spindle, continuous at their distad ends with the cilia and at their centrad ends with a fiber that can be traced quite to the neighborhood of the nucleus. I could not trace these fibers into the basal parts of the cells, except in one instance, and I could not be sure of that (Fig. 27a).

Another interesting observation in connection with the basal bodies is that they are bent in one direction on one side of the canal and in an opposite direction on the other side. In Fig. 27, which represents a longitudinal section of the endoderm and the supporting lamella of the dorsal (i. e. farthest from the eyes) side of the peduncle, the distal ends of the basal bodies are bent towards the ampulla, while on the ventral side they would be bent away from the ampulla. This seems to suggest that the flagella move the contents of the canal in one direction on the dorsal side of the canal and in an opposite direction on the ventral side. Conant observed in living material that bodies in the ampulla and the canal were moving about, and that bodies within the tentacles were moving in opposite directions at the same time. This last observation and the histological facts just described, I believe, are mutually corroborative. Again, a priori, we should expect some such mechanism as the one described to bring about an exchange between the contents of the ampulla and that of the stomach pockets. I have not as yet been able to demonstrate a similar flagellate mechanism in the tentacles. Flagella and basal bodies are present in the tentacles, but I could not determine that the basal bodies had any definite arrangement like that shown in Fig. 27. (See under tentacles.) I may add, yet, that the cells in the canal of the manubrium have cilia, similar to the ones just described, with large basal bodies, and with centrad continuations. Finally, I am not certain but that these cells form buds at their ends quite like those I describe for the endothelial cells of the tentacles (see below), and that they aid in the formation of the floating cells. I thought I saw such buds just at the entrance of the lumen of the peduncle into the ampulla, but could not find conclusive evidence.

The Tentacles and the Pedalia.—My observations on the tentacles were begun with the object of demonstrating a flagellate mechanism similar to the one described above for the endothelium of the peduncle. While I have failed to demonstrate such a mechanism for the tentacles, yet several interesting points came to my notice. It will be remembered that the tentacles of the CubomedusÆ are not directly attached to the bell, but that a blade-like portion, the pedalium, intervenes between the tentacles and the bell. For figures of the pedalia and the tentacles the works of Haake, Claus, Conant and Maas[22] may be consulted.

The Ectoderm.—The ectoderm of the tentacles is the seat of a number of differentiations. It is quite thick, as the figures (28 and 29) show, and in this respect is very different from the pedalia, on which the ectoderm cells are quite cubical. I found evidence of cilia here and there, but I can add nothing definite about them. Neither can I add any definite statements regarding the ectoderm cells proper, but what I have to say relates to their differentiations.

(a) The thread cells are of two kinds, larger ones and smaller ones. This is well shown in Fig. 29, which is part of a transverse section of a tentacle of Tripedalia. Two kinds of nettle-cells are also present in the tentacles of Charybdea, but they were specially well shown in Tripedalia. The structure of these thread-cells seems to be typical, and I have little more to say about them. I wish, however, to call attention to the five or six unstriped muscle-fibers that are attached to their basal lateral parts, and which connect them with the basement membrane (Figs. 28, 29). Claus describes these muscle-fibers and mentions that Fr. MÜller has described them before him, but I have not found them mentioned elsewhere in the literature of nettle-cells. Professor Brooks tells me, however, that he has often found them. It would appear from Fig. 29 that they serve to retract the thread-cells from the surface. Claus suggests that the muscles are developed from the cnidoblasts.

(b) The plain subectodermal muscle-fibers are of interest. In Charybdea they lie wholly enclosed within canals of the supporting lamella (Fig. 32, upper part). They run longitudinally, and near the base of each tentacle pass out of their canals and become strictly subectodermal (Figs. 31, 32). This is for Charybdea. In Tripedalia they rarely come to lie in closed canals as in Charybdea. These facts show beyond doubt that these muscles are developed from the ectoderm. Claus has suggested their ectodermal origin, but did not demonstrate it. He also suggested that they become inclosed in canals by the supporting lamella pushing up around them and finally fusing above them. This, I believe, is demonstrated by the conditions in Tripedalia (Fig. 29). Here the canals usually remain open, but occasionally, as in the left-hand canal, one may become completely inclosed. This condition of things suggests the intra-lamellar muscles found in actiniarians. The nuclei found in the canals with the muscle-fibers probably belong to the cells from which the muscles become differentiated. Claus figures these muscle-fibers and nuclei, and it may be added that the supporting lamella he figures, for C. marsupialis, is much thicker than I have figured it for C. Xaymacana and Tripedalia cystophora. The number of muscle-canals also is greater and occupies a much greater depth of the thickness of the lamella. Since Claus gives a figure of a transverse section showing the muscles in their enclosed canals, I have not deemed it necessary to duplicate his figure. In the transition from a tentacle to a pedalium, the muscles are most strongly developed toward and at the edges of the pedalium. This is true for the pedalia in general, and accounts for the readiness with which they can be bent inwards, as noted in the physiological part of this paper.

(c) I have found a single ganglion-cell among the cells of the ectoderm of the tentacles. This showed so plainly that I have figured it (Fig. 28). Other ganglion-cells no doubt exist, but could probably not be distinguished from other cells. In its position in Fig. 28 it appears to be associated with the nettle-cell shown just above it. Its position is very much the same as that figured by Lendenfeld (25a).

The Endoderm.—The cells of the endoderm of a tentacle are long and quite slender (Fig. 31). At their bases they are vacuolated quite like the cells of the ampulla and the canal of the sensory clubs. They contain a well-formed nucleus with a nucleolus. In their distal half small light bodies with a dark center are very evident. These bodies are evidently a secretion.

Another peculiar phenomenon presents itself in these cells. The distal part of each cell becomes separated off from its body by what appears to be the formation of a transverse cell-wall (Fig. 31, c-d). I have found the ends of these cells quite separated off in some series. The formation of the walls seems to begin as a thickening at the sides of the cells, and a section through this region, transverse to the cells, would appear like Fig. 30. The dots in the centers of the polygonal areas of this figure are the centrad continuations of the cilia to be described below. As already remarked in describing the endoderm of the ampulla, I believe we here have another place of origin of the “floating cells.” The secretion just described moves into the distal parts of the cells prior to their separation (Fig. 31). In some series I could see these secretion bodies much more numerous within the distal ends of the cells than in Fig. 31.

As will be seen in Fig. 31, each of the endoderm cells of the tentacles has a flagellum that extends into the lumen of the tentacle. Each flagellum has a thickening just within its cell, which may be regarded as a basal body. From this basal body, again, a small fiber extends centrad into each cell. It does not appear that the flagella are thrown off with the distal parts of the cells; at all events, I never found them connected with any of the floating cells except in a few doubtful instances.

What I have said for the endoderm of the tentacle of Charybdea applies equally to Tripedalia.

Claus, in his figure of a transverse section of a tentacle of C. marsupialis shows the endoderm as cubical. I cannot explain why there should be such a difference between the endoderm of the tentacles of C. marsupialis and that of the tentacles of C. Xaymacana and Tripedalia cystophora. Claus does not describe the endoderm in detail.

The endoderm cells of the pedalia of both Charybdea and Tripedalia are cubical and possess flagella, basal bodies, and centrad continuations, quite like those I have described for the endoderm cells of the ampulla. The double nature of the basal bodies and the centrad continuations is, however, not so evident. A secretion I did not find. Histologically, therefore, the endothelium of the pedalia corresponds rather with that of the ampulla, and that of the tentacles with that of the peduncle of the clubs.

SUMMARY.

The most important results in the histological part of this paper relate to the structure of the retinas of the eyes of the sensory clubs.

The retina of the distal complex eye is composed of three kinds of cells: two kinds of sensory cells (the prism and pyramid cells), and the long pigment cells (Figs. 1-9). The prism and pyramid cells have each an axial nerve fiber in their prisms and pyramids respectively. These fibers I could, however, trace only to the neighborhood of the nuclei. But since I could trace similar fibers in the retinal cells of the simple eyes (Fig. 16) past the nucleus into the subretinal nerve tissue, I believe that the axial fibers in question also extend centrad as nerve fibers into the subretinal nerve tissue. Other observers also figure such fibers as extending centrad as nerve fibers. The axial fibers of the prism cells have each a dumbbell-shaped basal body at their entrance into the pigmented part of a cell. The evidence for a body of such shape in the pyramid cells was not conclusive, though a basal body for the axial fiber exists. The long pigment cells project or retract their pigment in light or darkness respectively and thus seem to serve to check the diffusion of light in the retina. I have also supposed that these cells may serve for conducting impulses to the lens, and that the latter is adjustable.

The proximal complex eye (Fig. 13) has only the prism cells present in its retina, and not two kinds of cells as Schewiakoff has described (see text, pp. 53, 60, 63) for all the eyes.

The simple eyes (Fig. 12), two on each side of a club, four in all, also have only one kind of cells in their retinas, and each cell has a flagellum extending into the vitreous secretion of the lumen. These flagella could be traced centrad as a nerve fiber (Figs. 12, 16). Similarly, a nerve fiber could be traced centrad from the flagella of the epithelial cells of the clubs. Dumbbell-shaped basal bodies for the flagella of the simple eyes could also be demonstrated, but the evidence for this in the epithelial cells of the clubs was not so satisfactory.

Other points of interest are: A secretory epithelium lining the ampulla of the clubs, and a somewhat similar epithelium lining the canals of the tentacles (Figs. 7, 27, 31); the partial origin of the “floating bodies” in the canals of the clubs and tentacles and the stomach pockets from these epithelia (Figs. 18, 19); two flagella to each cell of the endothelium of the ampulla and of the pedalia (Figs. 7, 17); the peculiar nuclei in the endothelial cells of the ampulla (Fig. 20); the longitudinal muscles of the tentacles being completely inclosed within canals of the supporting lamella, but near the base of a tentacle becoming subectodermal. This demonstrates their ectodermal origin. In Tripedalia it is seldom that any of these muscles become enclosed as in Charybdea (Fig. 29).

If to the reader my results seem to embody a somewhat heterogeneous detail, he must remember that the work consists partly in corroborating and partly in supplementing the work of previous observers, and that, in general, histological detail does not usually make the most readable paper.

Biological Laboratory, Johns Hopkins Univ., May 1899.