It seems to be the popular opinion that sponges are essentially natives of the warmer seas, and it will probably be a surprise to many young amateur naturalists to learn that there are about three hundred species of this sub-kingdom of the animal world to be found on our own shores. It must not be thought, however, that they are all comparable with the well-known toilet sponges in regard to either size or general form and structure, for some of them are very small objects, no larger than about one-twentieth of an inch in diameter, and some form mere incrustations of various dimensions on the surfaces of rocks and weeds, often of such general appearance that they would hardly be regarded as animal structures by those who have not studied the peculiarities of the group.

Sponges are known collectively as the Porifera or Polystomata, and constitute a separate sub-kingdom of animals of such distinct features that they are not readily confused with the creatures of any other group. Their principal characteristic is expressed by both the group names just given, the former of which signifies ‘hole-bearing,’ and the latter ‘many openings’; for in all the members of the sub-kingdom there are a number of holes or pores providing a means of communication between the body cavity or cavities and the surrounding water. Most of these holes are very small, but there is always at least one opening of a larger size at the anterior end.

It will be seen from what we have just stated that sponges exhibit a distinctly higher organisation than the protozoa described in the last chapter, inasmuch as they possess a permanent body-cavity that communicates with the exterior; but in addition to this there are many points of differentiation of structure that denote a superior position in the scale of life.In order to ascertain the general features of a sponge we cannot do better than select one of the simplest forms from our own shores. If we place the live animal in a glass vessel of sea water, and examine it with a suitable magnifying power, we observe a number of minute pores scattered over its whole surface; and a much larger opening at the free end. The animal is motionless, and exhibits no signs of life except that it may contract slightly when touched. The water surrounding the sponge also appears to be perfectly still, but if we introduce some fine insoluble powder, such as precipitated chalk, or a drop of a soluble dye, the motion of the suspended or soluble material will show that the water is passing into the sponge through all the small pores, and that it is ejected through the larger opening.

On touching the sponge we observe that it is of a soft, gelatinous consistence throughout, or if, as is often the case, the body is supported by a skeleton of greater or less firmness, a gentle application of the finger will still show that this framework is surrounded by material of a jelly-like nature. This gelatinous substance is the animal itself, and a microscopic examination will show that its body-wall is made up of two distinct layers, the inner consisting of cells, many of which possess a cilium or whip-like filament that protrudes from a kind of collar, its free extremity extending into the body-cavity.

These minute cilia are the means by which the water currents just described are set up. By a constant lashing movement they urge the fluid contained in the body-cavity towards the larger hole, thus causing the water to flow in through the numerous small pores. This circulation of sea water through the body-cavity of the sponge is the means by which the animal is supplied with air and food. Air is, of course, absorbed from the water by the soft material of the external layer of the body, but the constant flow of fresh water through the body-cavity enables this process of respiration to go on with equal freedom in the interior. The mode of feeding of the sponge is very similar to that of the protozoa. Organic particles that are carried into the body-cavity, on coming in contact with the cells of the internal layer, are absorbed into their protoplasm by which they are digested. Thus the sponge may be compared to a mass of protozoon cells, all united into a common colony by a more or less perfect coalescing of the cell-substance, some of the units being modified in structure for the performance of definite functions. The air and food absorbed by any one cell may pass readily into the surrounding cells, and thus each one may be said to work for the common weal.

The description just given applies only to the simplest of the sponges, and we have now to learn that in the higher members of the group the structure is much more complicated. In these the surface-pores are the extremities of very narrow tubes which perforate both layers of the body-wall and then communicate with wider tubes or spaces within, some of which are lined with the ciliated cells above described. These spaces, which are sometimes nearly globular in form, and often arranged in groups with a common cavity, communicate with wider tubes which join together until, finally, they terminate in a large opening seen on the exterior of the sponge. Hence it will be seen that the water entering the minute pores of the surface has to circulate through a complicated system of channels and spaces, some of which are lined with the ciliated cells that urge the current onwards before it is expelled through the large hole. Further, imagine a number of such structures as we have described growing side by side, their masses coalescing into one whole, their inner tubes and spaces united into one complex system by numerous inter-communications, and having several large holes for the exit of the circulating water, and you then have some idea of the general nature of many of the more complex sponges to be found on our shores (see fig. 66).

But even this is not all, for as yet we have been regarding the sponges as consisting of animal matter only, whereas nearly all of them possess some kind of internal skeleton for the support of the soft, gelatinous animal substance. The skeleton consists of matter secreted by certain cells from material in the water and food, and is either horny, calcareous, or siliceous. The horny skeleton is formed of a network of fibres of a somewhat silky character, and often, as in the case of the toilet sponges, highly elastic; but it is sometimes so brittle that the sponge mass is easily broken when bent. The fibres of this framework support not only the outer wall of the sponge, but also the walls of all the internal tubes and spaces, which are often of so soft a nature that they would collapse without its aid.

The other forms of skeletons consist of minute bodies of carbonate of lime or of silica, respectively, which assume certain definite shapes, resembling stars, anchors, hooks, pins, spindles, &c., and are known as spicules. Such spicules are usually present in those sponges that have horny skeletons, but in others they form the entire skeleton.Sponges sometimes increase by division, a part being separated from the parent mass and then developing into a complete colony; and they may be reproduced artificially to almost any extent by this method, each piece cut off, however small, producing a new sponge. They also increase by a process of ‘budding,’ the buds produced sometimes remaining attached to the original colony, thus increasing its size, but on other occasions becoming detached for the formation of new colonies on a different site. In addition to these methods of reproduction there are special cells in a sponge that possess the function of producing eggs which are ejected through the larger holes. The eggs are usually developed in the autumn, and, after being ejected, swim about freely for a time, after which they become fixed to rocks or weeds, and produce sponges in the following year. The eggs may often be seen towards the end of the summer by cutting through a sponge, or by carefully pulling it asunder. They are little rounded or oval bodies, of a yellowish or brownish colour, distinctly visible to the naked eye, occupying cavities in the interior.

Sponges are classified according to the composition of the skeleton and the forms of the spicules, the chief divisions being:—

The first of these divisions contains about a dozen known British species, which are to be found on the rockiest shores, attached to stones, weeds, or shells, generally hidden in very secluded holes or crevices, or sheltered from the light by the pendulous weeds. They should be searched for at the lowest spring tide, particular attention being given to the under surfaces of large stones, narrow, dark crevices, and the roofs of small, sheltered caves. They may be readily recognised as sponges by the numerous pores on the surface, though these are often hardly visible without a lens, and the calcareous nature of the skeleton may be proved by dropping a specimen into dilute hydrochloric acid, when the carbonate of lime will speedily dissolve, the action being accompanied by the evolution of bubbles of carbonic acid gas.If calcareous sponges are to be preserved for future reference, they may be placed in diluted spirit, in which case the animal matter, as well as the mineral substance, will be preserved with but little alteration in the natural appearance and structure. A specimen which has been decalcified by means of acid, as above described, may also be preserved in the same manner; and small portions of this will serve for the microscopic study of the animal portion of the sponge. If the skeleton only is required, the sponge is simply allowed to dry, when the soft animal substance, on losing its contained water, will leave hardly any residue; or, better, allow the calcareous sponge to macerate in water for some days for the animal substance to decompose, and then, after a few minutes in running water, set it aside to dry.

Small portions of the skeleton, examined under the microscope, will show the nature of the calcareous spicules of which it is composed. These consist of minute needles and stars, the latter having generally either three or four rays.

We give figures of three of the calcareous sponges of our shores, the first of which (Grantia compressa) resembles little oval, flattened bags, which hang pendulous from rocks and weeds, sometimes solitary, but often in clusters. The smaller openings are thickly scattered over the flat sides of the bag, and the larger ones, through which the water is expelled, around the margin. When the sponge is out of the water and inactive, the two opposite sides of the bag are practically in contact, but, when active, the cavity is filled with water by means of the whip-cells that line it, and the sides of the sponge are then more or less convex.

The ciliated sycon (Sycon ciliatum), fig. 70, though of a very different appearance externally, is similar in structure to Grantia. It is also found in similar situations, and is not uncommon on many parts of the South Coast, from Weymouth westwards. The other example, Leucosolenia botryoides, shown in fig. 71, is a branching calcareous sponge, consisting of a number of tubes, all united to form one common cavity which is lined throughout with whip-cells. It is usually found attached to weeds.

Nearly all our British sponges belong to the group Demospongia—common sponges; but the members of this group present a great variety of form and structure. Most of them have a skeleton consisting of siliceous spicules, but some have a horny skeleton, somewhat after the nature of that of the toilet sponges; and others, again, have fleshy bodies entirely, or almost entirely, unsupported by harder structures. They are sometimes known collectively as the Silicia, for the greater number of them have skeletons consisting exclusively of siliceous matter, while the so-called horny sponges usually have spicules of silica intermingled with the horny substance, and even those which are described as having no skeleton at all sometimes contain scattered spicules of silex.

As the spicules of sponges are in themselves beautiful objects, and are important to the naturalist, inasmuch as they form a basis for the classification of sponges, it is well to know by what means they may be separated from the animal for microscopic examination. The separation is based on the fact that nitric acid (aqua-fortis) will destroy organic matter while it has not the slightest action on silica. In some of our common horny sponges the fibres are so transparent that, when teased out and placed under the microscope, the siliceous spicules may be seen embedded within them, but the spicules, both in these and the fleshy sponges, may be separated completely from the animal matter by putting a fragment of the sponge in a test-tube, covering it with nitric acid, and boiling it for a short time. The tube should then be filled up with water and allowed to stand undisturbed for a time, after which the liquid is poured off gently from the sediment. If the sediment is then put under the microscope on a slip of glass, it will be seen to consist of grains of sand, of which there is always a considerable amount in the pores and cavities of a sponge, and the siliceous spicules.Among the common objects of the sea shore is the horny skeleton of the sponge Chalina oculata, which is frequently washed on the beach by the waves, especially after storms. This sponge is not likely to be seen between the tide-marks except at the lowest spring tide, when it may be found suspended in a sheltered crevice or cave. The skeleton consists of a fine network of horny fibres, in the centre of which lie the spicules, imbedded in the horny material. The spicules are short and straight, tapering at both ends.

Fig. 73.—Halichondria panicea