The science of life : fully illustrated in tone and line and including many diagrams

LESS INDIVIDUALIZED ANIMALS

but sponges are unique in having an enormous number of separate openings through which they take in their food ; they have nothing that can be called a mouth.

There are many different kinds of sponges, all rooted to, or in, the bottom of the sea, river or pond that they inhabit, all making this slow, creeping current, and giving no other sign of life. There are common sponges that form green and yellow encrustations on rocks, crabs and so on, between tide-marks, others that occur in fresh watersometimes in such enormous masses that they may stop up waterpipes ; some, inhabiting the deep sea, weave glassy baskets of beautiful and symmetrical shapes instead of the horny web we wash with.

The sponge is a very loosely organized community of cells. It has a certain definiteness of architecture that varies from species to species, but its shape is by no means stereotyped ; individuals vary so much that the classification of the group is a matter of extreme difficulty. Besides the myriads of milling collar-cells that keep the current moving there are one or two other kinds of tissue—the muscle-like cells that control the diameters of the canals, wandering cells that drift about the gelatinous body of the sponge as they choose, recalling our leucocytes, the flat

are not differentiated into organs, and a fragment a tenth of an inch across would contain all the different kinds of tissue that form a sponge. When it is isolated it will simply continue to grow. Moreover, as with Obelia, a sponge can be forced through gauze so that all its cells are separated from each other; nevertheless, they will clump together again and rebuild a shoal of little sponges.

There are two methods of reproduction in this phylum. There is the sexual method: The spermatozoa are tiny cells with short,

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cells that pave its outer surface, and other Fig. 103. Five of the myriads of collared cells which keep

cells that make theskeleton. Thisskeleton may be of various kinds; there are sponges whose skeletons are made of tangled horny fibres, sponges whose skeletons are made of spicules of carbonate of lime, or of silica. These spicules are generally microscopic, and present a great variety of beautiful and symmetrical forms in different species. But the tissues of a sponge do not compare in complexity or diversity of organization with our own, or even with those of Obelia. It is even more undisciplined, even less individualized than the latter creature.

Like a Ccelenterate, a sponge has great powers of reorganization. A small piece cut away from a sponge will live and growand indeed, there is no very evident reason why it should not, for the parts of a sponge

the water in a living sponge in motion (highly magnified) .

blunt, stiff tails, and the eggs are giant cells that creep about the sponge like leucocytes ; fertilization results in the formation of a ciliated larva that swims away, settles down somewhere else and develops into a new sponge. In many sponges—especially in the fresh-water species—there is a second method by means of gemmules. A gemmule is simply a globular mass of tissue that withdraws itself, so to speak, from the life of the community to which it belongs and encloses itself, hermit-like, in a thick horny capsule. After resting for a time in the tissues of the sponge it is liberated, usually by the death and decay of the parent, and then proceeds to grow into a new sponge, just as any detached fragment may do.