Class III.—Sponges.
According to the observations of Mr. Carter, sponges begin their lives as solitary Amœbæ which grow by multiplication into masses, and assume endless forms according to the species; turbinate, bell-shaped, like a vase, a crater, a fan, flat, foliaceous and lobed or branching and incrusting the surface of stones. All the Amœbæ are so connected as to form one compound animal. The whole substance of a sponge is permeated by innumerable tubes which begin in small pores on the surface, and continually unite with one another as they proceed in their devious course to form a system of canals increasing in diameter and ending in wide openings called oscula, on the opposite side of the mass. Currents of water enter through the pores on the surface, and bring minute portions of food which are seized upon by a vast multitude of Amœbæ with long cilia which form the walls of the tubes and canals; and after they have extracted the nutritious part, the offal is carried into the sea through the oscula, by the current of water whose flux is maintained by the vibrations of the cilia. In the compressed and many of the tubular sponges the water passes through them in a straight line; in branched and encrusting sponges, the afferent and efferent openings are on the same surface. The water is inhaled continuously and gently like an animal breathing, but it is rapidly and forcibly ejected; and in its passage it no doubt furnishes oxygen to aërate the juices of the compound animal, whose flesh or sarcode is irritable while alive, and which has the power to open and shut the pores and oscula of the canals, for the whole sponge forms one compound creature whose mass is nourished by the myriads of Amœbæ of which it is constituted.
Within the animated sarcode mass of the sponges there is in most cases a complicated skeleton of fibrous network, either horny, calcareous, or siliceous, which supports the soft mass, and determines its form.
Besides the skeleton, the mass of sponges is for the most part strengthened and defended by siliceous, and more rarely by calcareous, spines or spicules, either imbedded among the fibres of the skeleton, or fixed to them by their bases. The fibres of the skeleton network always unite, whether they be horny, calcareous, or siliceous; the spicules never, though they often lie in confused heaps over one another. They are of innumerable forms and arrangements. Some are like long needles lying close together in bundles, pointed or with a head like a pin at one or both ends; a great number are stellate with long or short rays; there may even be several different forms in the same sponge. Many calcareous sponges have cavities full of organic matter; and when the calcareous matter is dissolved by dilute acid, the organic base is left.
The common commercial sponges have a skeleton which consists of a network of tubular, horny, tough, and elastic fibres which cross in every direction. They have no spicules or very few; and when such do project from the horny skeleton, they are generally conical, attached by their bases, and their surface is often beset with little spines arranged at regular intervals, which gives them a jointed appearance. The common sponge which is so abundant in the Mediterranean has many forms; those from the coast of North America are no less varied, but that most used in the United States is turbinate, concave, soft, and tomentose.
Fig. 103. Section of Sponge.
In the calcareous sponges a mass of three-rayed spicules surround the interior canals, where they are held together by a cartilaginous substance which is wanting in the horny sponges, but which remains in this order after the destruction of the more delicate matter when the sponge is dried.[[12]] The pores are also occasionally defended by the projecting points of half buried spines.
In nearly every species of this order the pores on the surface are protected by spicules; and they are also projected from the surface of the large cloacal cavity, and curved towards its opening, to defend it from Annelids and other enemies.[[13]] Some species have a long ciliary fringe at the orifice of the cavity, through which the water may pass out, but no animal can come in.
The spicula and skeleton of most of the marine sponges are siliceous and singularly beautiful; the skeleton of the Dactylocalyx pumiceus of Barbadoes is transparent as spun glass; and a species from Madagascar has numerous simple transparent and articulated spicules implanted in the siliceous fibres of the skeleton. The Cristata, Papillaris, Ovulata, and many more have siliceous skeletons, some garnished with spicules of various forms, and the surface occasionally covered with a layer of siliceous granules.
The variety in the size, structure, and habits of the marine sponges is very great: temperate and tropical seas have their own peculiar genera and species; some inhabit deep water, others live near the surface, while many fix themselves to rocks, sea-weeds, and shells, between high and low water mark. There are very few dead oyster, whelk, scallop, and other shells that escape from the ravages of the Cliona, an extremely minute burrowing sponge of the simplest structure, which has a coat of siliceous spicules supposed to be the tools with which it tunnels a labyrinth through the mid-layer of the shell, in a pattern that varies with the species of the sponge. A communication is formed here and there with the exterior by little round holes, through which the sponge protrudes its yellow papillæ. From the force exhibited by this little sponge, it may perhaps be inferred to possess a rudimentary muscle and nerve.[[14]]
Sponges are propagated twice in the year by minute ciliated globules of sarcode, detached from the interior of the aquiferous canals, which swim like zoospores to a distance, come to rest, and lay the foundation of new sponges. The little yellow eggs of Halichondria panicea are lodged in the interstices between the interior canals; when mature, they are oval and covered with cilia, and are carried out by the currents; and after swimming about for some days fix on a solid object, become covered with bristles, spread out into a transparent film, charged with contractile vesicles of different sizes in all degrees of dilatation and contraction, as well as with sponge ovules. Spicules are developed at the same time, and these films ultimately become young sponges, and if two happen to meet they unite and are soldered together.[[15]] Besides eggs, larger bodies covered with radiating spicules are produced, containing granular particles of sarcode, each of which when set free by the rupture of the envelope, becomes an Amœba-like creature, and ultimately a sponge.
Fresh-water sponges are sometimes branched, and sometimes spread over stones, wood, and other substances; and one species covers an earthy mass some inches thick formed by its own decayed matter. The skeleton of such species as have one, consists of bundles of siliceous spicules, held together and mixed with groups of needles, the rods of which project through the surface of the sponge and render it spinous. The motions in the gelatinous sarcode mass are the most remarkable feature in the fresh-water sponges, which all belong to the genus Spongilla. Mr. Carter observed that portions of the surface of some individuals of the Spongilla fluviatilis in his aquarium had long cilia by means of which they rapidly changed their places during the spring, but when winter came they emitted processes from such parts of their surfaces as were free from cilia and retracted them again just like Amœbæ. These portions often had cells, and when the Amœba-like motions ceased, a nucleus and nucleolus appeared within them, and at last the whole gelatinous sarcode mass consisted of these cells or globules. Some had no nucleus, but were filled with green or colourless granules.
At certain seasons of the year, whatever the form of the fresh-water sponges may be, a multitude of minute hard yellow bodies are produced in their deeper parts. They consist of a tough coat containing radiating spicules like a pair of spoked wheels united by an axle with a pore in its surface. Within this last there is a mass of motionless granular cells, and when put into water the cells come out at the pore and give rise to new sponges.
Insulated groups of germs covered with cells called swarm-cells seem to form parts of the sponges; they lie completely within the mass of the living sponge. They have the form of a hen’s egg, are visible to the naked eye, and when they come into the water they swim in all directions for a day or two; become fixed; a white spot within is enlarged; and the constituents of young sponges appear.[[16]]
The generic forms of fossil sponges augment in number and variety from the Silurian to the Cretaceous beds, where the increase is rapid; but all the sponges which had a stony reticulated form without spicules passed away with the Secondary epoch, so that the family has no representatives in the Tertiary deposits or existing seas. The calcareous sponges which abound in the Oolite and Cretaceous strata, and attain their maximum in the Chalk, are now almost extinct, or are represented by other families with calcareous spicules. Siliceous fossil sponges are particularly plentiful. In England extensive beds of them occur in the Upper Greensand, and in some of the Oolitic and Carboniferous Limestones; and some beds of the Kentish Rag are so full of their siliceous spicules, that they irritate the hands of the men who quarry them. Since every geological formation except the Muschelkalk is found in England, the number and variety of fossil sponges are very considerable. The horny sponges are more abundant now than they were in the former seas. According to M. D’Orbigny the whole number of fossil sponges known and described amount to thirty-six genera and 427 species, which is probably much below the real number.[[17]]