THE SP0N&E8. 



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 themselres 

 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 being 

 simply horny or fibrous, hav- 

 ing few flinty or silicious spic- 

 ules. The Mediterranean sponges are the best, being the 



Fig. 18.— OS, OECulum; p, pore or 

 mouth, towards which the outer 

 arrows point; c, ciliated chamhers. 



Fio. 19.— Development of a sponge (Syconr<mhanus). A, ripe egg; B. stage with 

 four segmentation-cells ; 6', morula stage ; iJ, blastosphere, with large dark gran - 

 nlar cells (yc) at the open pole ; E, free-swimming larva, one haU of the body 

 (endodermal) being formed of long ciliated cells, the other (ectodermal) of large 

 granular cells. The mesoderm develops later. (All highly magnified.; 



