THE SPONGES. 



23 



vides into two, four, eight, sixteen, and more spheres, until 



it looks like a mulberry (Fig. 



19, 6'). The cells further mul- 



tiply, and arrange themselves 



into an outer (ectoderm) and 



inner layer (endoderm). Some 



of the cells are ciliated, and in 



this state the germ (E) leaves the 



parent sponge and swims about 



in the sea, finally fixing itself 



to some seaweed or rock. 



Of the marketable sponges 

 there are six species, with nu- 

 merous varieties. They are 

 available for our use from bein 



hnrnv nr filirnnc linv FlG - 18. os, opculum; p, pore or 

 my Or 1U10US, liav- mouth) towards which the outer 



ing few flinty or silicious spic- arrows P int ; c ' ciUated chambers - 

 ules. The Mediterranean sponges are the best, being the 



glc 



FIG. 19. Development of a sponge (Si/ranraphnnus"). A, ripe egg; B, stage with 

 four segmentation-cells; C, morula stage; 7), blastosphere, with large dark gran- 

 ular cells (t/c) at the open pole; A', free-swimming larva, one half of the body 

 (endodermal) being formed of long ciliated cells, the other (ectodermal) of lai^e 

 granular cells. The mesoderni develops later. (All highly magnified .) 



