Although the Mollusca do not come within the limits of this work they nevertheless afford objects worthy of microscopic investigation. The gills of a bivalve mollusk are like crescent-shaped leaves fixed by their stalks to the transverse extremities of the mantle, so that the greater part floats freely in the water.

To the naked eye the gills appear to be formed of radiating fibres of admirable structure; but the microscope shows that each leaf consists of a vast number of straight transparent and tubular filaments, arranged side by side so close that 1,500 of them might be contained in the length of an inch. These filaments, however, apparently so numerous, in fact consist of only one exceedingly long filament in each gill, bent upon itself again and again throughout its whole length, both at the fixed and free ends of the leaf. These long filaments are fringed on both sides by lines of cilia continually vibrating in contrary directions. By this action a current of water is perpetually made to flow up one side of the filaments and down the other, so that the blood which circulates in their interior is exposed throughout their long winding course to the action of oxygen in the water. The duration of these vibrations in the mollusca is marvellous. The cilia on a fragment of a gill put into water by Mr. Gosse fifteen hours after the death of the mollusk caused a wave to flow uniformly up one side of the filaments and down the other. Even twenty-hours after the death of the animal the ciliary motion was continued on such parts as were not corrupted, a remarkable instance of the inherent contractility of the animal tissues.

The refined mechanism of the gills of the common Mussel enables it to live when attached to rocks above high-water mark, so as only to be immersed at spring tides. By the movements of cilia, water is retained in the gill-chamber, which derives oxygen from the atmosphere, and animalcula supply the Mussel with food.

The mollusks that burrow in sand or mud have two tubes fringed with cilia, which they protrude into the water above them. The water which is drawn into one of these tubes by the action of the cilia passes in a strong current over the gills, aËrates the blood, brings infusorial food for the animal, and is expelled in a jet from the other tube. The foot at the other extremity of the shell is the organ with which the mollusk makes its burrow in sand, clay, chalk, stone or wood.^[41]

The common Cockle digs into the sand, and uses its foot both for digging and leaping; it is cylindrical, and when the Cockle is going to leap, it puts out its foot and bends it into an elbow; then having fixed the hooked point firmly in the sand, by a sudden contraction of the muscles it springs to a considerable height and distance, and leaps actively along the surface of the sand. The lowest part of fig. 166 is a magnified section of the foot, showing the muscular system which gives the animal that power. It consists of many rows of longitudinal muscles, interlaced at regular distances by transverse fibres. When the foot is extended, the Cockle has the power of distending it by filling a network of capillary tubes with water till it is almost transparent. The water is also distributed through the body and into the gill-chamber, which opens and shuts every ten minutes or oftener, in order to maintain the supply; and it has egress through the pores in the mantle and foot, for some burrowing mollusks squirt it out through the foot when disturbed. This water-system is unconnected with the circulation of the blood.

Each bivalve mollusk is both male and female; and the fertilized eggs pass into the gills of the parent, where they undergo a kind of incubation. At a certain time the yellow yolk of the egg is divided into a granular mass, which separates from the liquid albumen and produces cilia. The cilia cause the albumen to revolve round the interior of the egg; at last the granular mass revolves with it, while at the same time it rotates about its axis in a contrary direction at the rate of six or eight times in a minute. When still in the egg, all the organs of the little embryos are formed in succession, even the little valves of the shells are seen to open and shut, but the embryos are hatched before they leave the parent, and swim about in the cavity of the external gill.

Shells of the Mollusca.

When these mollusks come into the water, they soon find their transparent white shell too small, and begin to increase its size by means of the mantle, which is an exquisitely sensible fleshy envelope applied to the back of the animal, extending round its sides like a cloak, only meeting in front, and it is for the most part in close contact with the whole interior of the shell. Its edges are fringed with rows of slender tentacles, and studded with glands, which secrete the colours afterwards seen in the shell; the glands in the rest of the mantle secrete only colourless matter.

When the animal begins to enlarge its shell, it attaches the borders of the mantle to the margin of the valves, secretes a film of animal matter, and lines it with a layer of mucus containing carbonate of lime and colour in a soft state, which soon becomes hard, and is then coated internally by the other glands of the mantle with colourless carbonate of lime.

The two strata thus formed, one richly coloured, the other white, often nacreous and brilliantly iridescent, are highly organized substances. Examined with the microscope, they present remarkable varieties in some of the natural groups of bivalve Mollusca; the structure of the Monomyarian Oyster is characteristic of the division which has but one muscle; the Dimyaria, having two muscles, are represented by the Cockle.

The exterior laminÆ at the edge of the fragile valves of a Pinna are often so thin and transparent that the organization of the shells may be seen with a low magnifying power. A fragment has the appearance of a honeycomb on both surfaces (fig. 167), whereas its broken edge resembles an assemblage of basaltic columns. The exterior layer of the shell is thus composed of a vast number of nearly uniform prisms, usually approaching to the hexagonal structure, whose lengths form the thickness of the lamina, their extremities its surfaces. When the calcareous part of the lamina is dissolved by dilute acid, a firm membrane is left, which exhibits a hexagonal structure (fig. 168), as in the original shell; but it is only in the shells of a few families of bivalves nearly allied to the Pinna that this combination of the organic and mineral elements is seen in this distinct form; it is beautifully displayed in the nacreous shells.

In many shells the internal layer has a nacreous or iridescent lustre, shown by Sir David Brewster to depend upon the striation of its surface, by a series of nearly parallel grooved lines. When Dr. Carpenter had dissolved the calcareous matter from a thin piece of nacreous substance, taken from the shell of the Haliotis splendens, or Ear Shell, there remained an iridescent membrane, which presented to the microscope a series of folds or plaits somewhat regular, and splendidly iridescent, but when the plaits were unfolded and the membrane stretched, the iridescence vanished. So the varied hues of mother-of-pearl are owing to the folds of an organic membrane.

The shells of the Gastropoda, or crawling mollusks, have a structure peculiar to themselves, but by no means so much varied as that of the bivalve class. The Strombus gigas, or Queen Conch, the Cassis, or Helmet Shell, and the beautiful porcellanous CyprÆÆ or Cowries, are much valued by the artists who cut cameos, on account of the structure of their shells, which consists of three strata, the same in composition, but differing in arrangement, and sometimes in colour. Each stratum of the shell is formed of many thin laminÆ, placed side by side, perpendicular to the plane of the stratum, and each lamina consists of a series of prismatic spicules with their long sides in close approximation; the laminÆ of the inner and outer strata have their spicules parallel to one another, while the spicules of the intermediate lamina are perpendicular to those on each side. According to Dr. Bowerbank, who discovered this complicated structure, the spicules are microscopic tubes filled with carbonate of lime.

The Spondylus gÆdaropus has sixty ocelli constructed for accurate vision. One can form no idea of the effect of so many eyes, unless they combine to form one image as our eyes do. The common Pecten, or Scallop, pretty both in form and colour, has a number of minute brilliant eyes arranged along the inner edge of the mantle, like two rows of diamond sparks. Some families of mollusks are destitute of eyes, even of the simplest kind; and it has been observed that those mollusks most liberally provided with eyes are also endowed with the most active and vigorous motions. The bivalves do not appear to have either taste or organs of hearing, but they are exceedingly sensitive to touch. It is singular that animals which have neither head nor brain should have any senses at all. A nerve-collar round the gullet with a trilobed nerve-centre on each side supplies the place of a brain; nerves extend from these; besides there are nerve-centres in various parts of the unsymmetrical bodies of the acephalous mollusks.

The Gastropoda, or crawling mollusks, have a head, and are consequently animals of a higher organization than the Conchifera or bivalve class. Their mantle forms a vaulted chamber over the head and neck, and envelopes the foot or crawling-disk; all these the animal can protrude or draw in at pleasure. The head is of a globular form, with two or four exceedingly sensitive tentacles, arranged in pairs on each side of it, as in the garden snail, which has four, two long and two short. These tentacles, which the snail can push out and draw in at pleasure, are hollow tubes, the walls of which are composed of circular bands of muscle. The tentacles are pushed out by the alternate contractions of these circular bands, but they are drawn in again like the inverted finger of a glove by muscular cords proceeding to the internal extremity of the tentacle from the muscle that withdraws the foot. The structure of the tentacles is the same in all the crawling mollusks; they are most sensitive in the Helix or Snail family, but they are believed to be delicate organs of touch in all.

The Gastropod mollusks never have more than two eyes, either placed on the tips, or at the base of one pair of tentacles; in the snail they may be seen as black points on the tips of the longest pair. In some of the higher Gastropods they are of great beauty, and appear to be perfectly adapted for distinct vision. Organs of hearing were discovered by Dr. Siebold at the base of one of the pairs of tentacles, consisting of vesicles containing a liquid and calcareous otolites, which perform remarkable oscillations due to the action of vibratile cilia. In the Snail and Slug group the number of otolites varies from eighty to one hundred.

The mouth of a Gastropod is a proboscis, with fleshy lips, generally armed with horny plates or spines on the jaws. The Snail has a crescent-shaped cutting plate on its jaw, and a soft bifid lip below, but the tongue is the most remarkable microscopic object in this group of Mollusca. In the terrestrial Gastropods, it is short and entirely contained within the nearly globular head. It is tubular behind, but in front it is spread into a nearly flat narrow plate, traversed by numerous rows of minute recurved teeth, or spines set upon flattened plates; in the Garden Snail or Slug each principal tooth has its own plate. Fig. 170 represents a magnified portion of a Snail’s tongue by Dr. Carpenter; the rows at the edge are separated to show the structure. The teeth are set close one to another, and are often very numerous. In the Helix pomatia, a snail found in the middle and southern counties of England, they amount to 21,000, and in the great slug (Arion ater), there are 26,800. This kind of tongue only serves for rasping vegetable food. All the TrochidÆ, which are marine mollusks that are supposed by some naturalists to live on fuci, are remarkable for the length and beauty of their narrow spiny tongues. Fig. 171 is a small portion of the tongue or palate of the Trochus zizyphinus, highly magnified; the large teeth of the lateral bands, as well as the small teeth in the centre, have minutely serrated edges. Fig. 172 shows the Trochus granulatus in the act of crawling.

The Limpet lives on sea-weeds. The animal is large in proportion to the size of its conical shell; it has a long leaf-shaped gill under the edge of the mantle. The head has a short proboscis and pointed tentacles, with eyes at their base. The mouth has a horny jaw and a very long tongue, moved by muscles rising from firm objects on each side of it. Fig. 173 represents the tongue beset with recurved hooks, and A shows a portion highly magnified. These recurved teeth are transparent, amber-coloured, and in the Limpet, as in most of the other Gastropods, they are chitinous. The teeth towards the point of the tongue are sufficiently hard to rasp the food; and it is said that when they are worn down, the part of the tongue supporting them falls off, and that the waste is supplied by a progressive growth of the tongue from behind, and a hardening of the teeth in front. The soft reserved portion is coiled in a spiral when not in use.

All the species of PatellidÆ, or Limpets, have the power of making cavities with their foot in the surface of the rocks to which they adhere. The cavity exactly corresponds in shape and size with the mouth of the shell, which is sunk and very strongly glued into it, yet the Limpet dissolves the glue with a liquid secretion, roams in quest of food, and returns again to its home: both fluids are secreted by a multitude of glands in the foot, which is the instrument of adhesion.

The tongue of the carnivorous Gastropods is a very formidable weapon, used for boring holes in the hardest shells. The round holes in dead shells frequently met with on our coasts show that their inhabitants had fallen a prey to some of these zoophagous Mollusks. The tongue of these predatory Gastropods is a narrow mechanical file, sometimes twice or even three times the length of the whole animal, and when not in use it is curled up near the foot. It is spined in various microscopic patterns according to the genus, and is supported by two firm parts from whence the muscles spring that work the rasp.

The Periwinkles have a ribbon-shaped tongue, rough with hooked teeth; the ScalariÆ have also predatory tongues, but of all the Gastropod mollusks, the Whelk and its numerous allies are the most predacious. The Purpura or Dog Whelk especially is the most ravenous of mollusks. Its long tongue is armed with hooked and spined teeth, placed three in a row; with this weapon and a proboscis capable of boring, they have been known to exterminate a whole bank of Mussels.

The Common Whelk is represented in fig. 174. When in the act of crawling, its head, with two tentacles, is at one extremity, its foot at the other, sometimes used as an organ of prehension; and it has a siphon for carrying water to the gills at the end of the shell.

All the families of the naked mollusks or Sea Slugs, furnish beautiful objects for the microscope. The two sexes are united in the same individual, and in their embryonic state they have a shell, which is cast off long before they come to maturity. The gills placed on the naked body are capable of being withdrawn into a cavity in the medial line of the back, and are either plumose, or like the leaf of a plant pinnated again and again, but they vary in form and position in the different genera.

In the group of the EolidÆ, the gills are like leafless trees in most genera, but in the principal genus Eolis, they are long, spindle-shaped, sharp-pointed papillÆ, arranged in transverse rows or clusters along the sides of the back, leaving a space between them, as in fig. 175. They are covered with long cilia, whose vibrations send a perpetual current of sea-water along each of them, the respiration is aided by vibrating cilia, scattered almost over the whole body, and the circulation of the blood is very simple.

The Eolis has a head prolonged into a pair of tentacles which are active and as sensitive as antennÆ. Another pair on the back have ten or twelve narrow plates twisted in a spiral round them; the eyes are at the base of these horns. The mouth contains horny jaws and a spiny tongue like a mere strap covered by numerous transverse plates armed with recurved spines not more than a sixth part the thickness of a human hair. Fig. 176 represents the tongue and some of the spines greatly magnified. The mouth leads into a short and large membranous stomach, from each side of which branches are sent off, from whence long canals traverse the papillÆ longitudinally, and perform the part of a liver. In many species these tubes are brilliantly coloured, but none are more beautiful than those in the Eolis coronata, which is found under stones, like a mass of jelly, not larger than a pea, at low spring tides, on our own coasts. When put into sea-water it expands till it is about an inch long (fig. 175). It is then pellucid, tinged with pink, and the central tubes in its numerous papillÆ are of a rich crimson hue, their surface reflects a metallic blue, and their tips are opaque white; as the animal keeps its papillÆ in constant motion the effect is very pretty.

The Eolis coronata, like all its congeners, has a stinging apparatus, consisting of an oblong bag, full of thread cells, placed at the extremity of each papilla, from whence darts can be ejected through an aperture in the tip. The whole of the EolididÆ are carnivorous, fierce, and voracious, setting up their papillÆ like the quills of a porcupine when they seize their prey; they tear off the papillÆ of their weaker brethren, and even devour their own spawn, though their chief food consists of zoophytes.

The Pteropoda, or wing-footed mollusca, are very small; they are incapable of crawling or fixing themselves to solid objects, but they are furnished with two fins like the wings of a butterfly, with which they float or row themselves about in the ocean, far from land in vast multitudes. The shell of the typical species HyalÆa (A, fig. 177), which resembles the thinnest transparent glass, consists of two valves, one, which is placed on the front of the animal, is long, flat, and ends in three points; the other valve, which is applied to the back, is short and convex, and in the lateral fissure between the two, the mantle is protruded. The head and fins project from an opening at the top of the shell. The fins, which are formed of muscular fibre, are fixed on a short thick neck, with the mouth lying between them, containing a tongue crossed by rows of long reversed teeth. The head has no tentacles, and the animal appears to be blind, but it has an auditory vesicle lined with cilia, which keeps a few otolites in motion. This little animal is highly organized; it has a gullet, a kind of crop and gizzard, a liver, a respiratory tube, a heart, a circulating and nervous system, which enables it to swim with a flapping motion of its fins.

The Clio pyramidata (B, fig. 177) is an elegant animal belonging to the same class. Its fragile transparent shell has the form of a triangular pyramid; and from its base proceeds a slender spine, and a similar spine extends from each side of the middle of the shell. The posterior part of the body is globular and pellucid, and in the dark it is vividly luminous, shining through the glassy shell. The fins of the HyalÆa and Clio or Cleodora are of a bright yellow, with a deep purple spot near the base. Both are inhabitants of the ocean.

The Clione borealis (fig. 178), which exists in millions in the Arctic Seas, is the most remarkable instance of the Naked Pteropods. It has neither shell nor mantle; its membranous body is not more than half an inch long, its head is formed of two round lobes, on each side of the neck there is a large muscular wing or fin; in swimming the animal brings the tips of the fins almost in contact, first on one side of the neck and then on the other. In calm weather, they come to the surface in myriads, and quickly descend again. There is a pair of slender tentacles close to the head, which are organs of feeling, a pair of eyes are placed on the back of the neck, and acoustic vesicles lined with cilia keep otolites in motion. Besides these organs of sense, the Clione has respiratory, digestive, and nervous systems. The latter consists of a nerve-collar round the gullet, with two nerve masses in its upper part, so the Clione is well supplied with nerves.

Upon each of the two round lobes of the head, there are three tentacles, totally different from those of feeling. They are, in fact, organs of prehension, which can be protruded or withdrawn at pleasure into a fold of the skin. When protruded, these six tentacles form a radiating crown round the mouth, which is terminal, and furnished with fleshy lips. Each of these tentacles is perforated by numerous cavities, appearing like red spots to the naked eye; however, Professor Eschricht discovered that each spot consists of a transparent sheath, enclosing a central body composed of a stem terminated by a tuft of about twenty microscopic suckers, capable of being thrust out to seize prey. The whole number of these prehensile suckers in the head of one Clione was estimated by Eschricht to amount to 330,000. Notwithstanding the vast prehensile power and multitude of these animals, they find abundance of food in the Arctic Ocean, for although the water is generally of the purest ultramarine blue, one fourth of the Greenland Sea, extending over 10° of latitude and some hundred feet deep, is green and turbid, with a profusion of minute animal life. The indefinite increase of the Clione borealis is checked by the whales, who feed upon them, and other minute inhabitants of the Arctic Seas. The Pteropods first appear in a fossil state in the Lower Silurian strata.

The Naked Cephalopods have an internal skeleton instead of a shell, in the shape of a transparent horny pen in the Calamary, or the well-known internal shell of the Cuttle Fish; they are divided into Octopods and Decapods, according to the number of their tentacles: the Poulpe, or Octopus vulgaris, is a type of the first, the Sepia or Cuttle Fish, fig. 179, and the Loligo vulgare or Squid, are types of the last. These creatures may be seen on rocky coasts, or in the ocean hundreds of miles distant from land. They are nocturnal, gregarious, carnivorous, and fierce,—their structure enables them to be tyrants of the ocean. They are strange-looking, repulsive creatures, with staring bright-coloured eyes, while crawling awkwardly on their fleshy arms head downmost; yet they are the most highly organized of all mollusks.

They have a distinct brain, enclosed in a cartilaginous skull, and all their muscles are attached to cartilages. The lower part of their body is surrounded by a mantle, which extends in front to form a gill chamber, in which there is a pair of plume-like gills; a funnel or siphon projects from the gill chamber immediately below the tentacles. All the naked Cephalopods propel themselves back foremost in the sea, by the forcible expulsion of water from the gill chamber through this siphon.

The head protrudes from the top of the mantle; it has a pair of large eyes on sockets, and some species of these animals have eyelids. The ears are cavities under the cartilage of the skull, containing a small sac and an otolite. The mouth, which is terminal, and surrounded by the tentacles, has powerful jaws like a parrot’s beak reversed, acting vertically. The tongue is large, the posterior part is covered with recurved spines, and the organ of smell is a cavity near the eyes. The Naked Cephalopods are remarkable for having three hearts, or propelling vessels—one for the circulation of arterial blood through the body, the others for projecting venous blood through the gills, at whose base they are situated.

The arms or tentacles of all the Naked Cephalopoda are formidable organs of defence and prehension, but are most powerful in the Loligo vulgaris, the Poulpe, and the Cuttle, on account of one pair of the tentacles being long slender arms, dilated at their extremities into flat clubs. On the inner surface of each of the tentacles, and upon the lower surface of the dilated extremities of the long ones, there are multitudes of sucking disks, which, once fixed to an object, adhere so firmly that it is easier to tear off a portion of the animal’s tentacle than to make it release its hold. These sucking disks, which are placed in parallel rows, are represented magnified in fig. 180. Each sucker consists of a firm cartilaginous or fleshy ring (e), across which a disk of muscular membrane (f) is stretched, having a circular opening (g) in its centre. A cone-shaped mass of flesh fills the opening like a piston, capable of being drawn backwards; the membranous disk can also be drawn in. When one of these sucking-disks touches a fish, the fleshy piston is instantaneously retracted, a vacuum is formed, and the edges of the disk are pressed against the victim with a force equal to the pressure of the superincumbent water and that of the atmosphere. The fish is powerless when embraced by the eight tentacles and their hundreds of suckers; but, if large enough still to struggle, the force is increased by drawing in the membranous disk. The Poulpe, the most powerful of the group which swims far from land, and has to contend with large slippery fishes, has a hooked claw in the centre of each sucking-disk, which is clasped into the fish the instant the vacuum is formed. The expansions at the extremities of their two long arms, which are thickly and irregularly beset with hooked sucking-disks, not only drag the fish into the embrace of the eight short tentacles, but they clasp round it and interlock, so that the fish can be torn to pieces by the parrot-like jaws, and eaten at leisure. The tentacles, long and short, have strong nerves, and a little nerve-mass occupies the centre of each sucking-disk, which gives the tentacles great power.

The sepia, or inky liquid, which all the Naked Cephalopods possess as a means of defence, is secreted in a pyriform bag, which has an outlet near the respiratory siphon. If the animal be alarmed when devouring its prey, it instantly lets go its hold, discharges the inky liquid into the water, and escapes unseen.

The skin of this class of animals is thin and semi-transparent; the surface immediately below it consists of numerous cells, of a flattened oval or circular form, containing coloured particles suspended in a liquid. The colour is seldom the same in all these cells; the most constant kind corresponds with the tint of the inky secretion. In the Sepia there is a second series of cells, containing a deep yellow or brownish colour; in the common Calamary, or Squid, there are three kinds of coloured cells—yellow, rose-coloured, and brown; and in the Poulpe there are red, yellow, blue, and black cells. The cells possess the power of rapid contraction and expansion, by which the coloured liquid is drawn into deeper parts of the surface, and is again brought into contact with the semi-transparent skin—thus constantly varying. In consequence of the high development of the nervous system, the skin of the Naked Cephalopods is of extreme sensibility; a mere touch brings a blush on that of the Poulpe, and they all assume the colour of the surface on which they rest as readily as the chameleon. Many of these nocturnal animals are luminous, and are easily attracted by bright metallic objects.