CHAPTER XXIV 

 THE SPONGES 



The ordinary bath sponge is the skeleton of an animal that 

 lives in the sea. Formerly sponges were considered plants 

 because of their irregular and plantlike habits of growth, but 

 their animal nature was finally established about 1857. 



The fresh-water sponge lives in ponds and streams and may 

 be found attached to the undersurface of rocks, dead leaves, 

 or sticks (Fig. 127, A). It forms incrustations a fraction of an 

 inch thick, or compact masses, and is gray or green in color. 



Unfortunately the fresh-water sponge has a very complicated 

 structure (Fig. 128, C) and is therefore not suited for laboratory 

 work. 



A Simple Sponge. — Most of the sponges live in the sea, and' 

 some of these are quite simple. For example, Leucosolenia 

 (Fig. 127, B), which grows on the rocks just below low-tide mark, 

 consists of a tube with side branches. The way the various 

 physiological processes are carried on may be explained by means 

 of Figure 128, A. One end of the sponge is fastened to the 

 rock; the other end contains an opening, the osculum (esc). 

 The cells lining the gastral cavity (G.C) are provided with whip- 

 like projections called flagella (Fig. 129, D), which beat back 

 and forth and create a current of water just as do the cilia in 

 the mussel (see p. 148). This water is drawn in through pores in 

 the body wall (Fig. 128, p) and passes out through the osculum 

 in the direction of the arrows shown in the figure. Food par- 

 ticles are drawn into the gastral cavity with the water and en- 

 gulfed by the cells. Waste matters pass out through the osculum 

 suspended in the water. Oxygen is taken in by the body wall 



