172 ZOOLOGY 



ducing an oval blastula which swims freely by means of cilia 

 or flageUa. While there are some peculiar features about the 

 gastrulation, a gastrula or two-layered embryo is ultimately 

 formed. Strangely enough it is the flagellate cells of the larva 

 that become the inner or gastral layer of the adult. Conversely, 

 the cells which in other Metazoa give rise to the gastral layer 

 form the outer epithelium of sponges. 



The embryo finally settles to the bottom and becomes 

 attached by the end containing the blastopore, which thus 

 becomes obliterated (Fig. 77, bl). An excurrent pore breaks 

 through at the opposite end, and the numerous incmrent 

 pores are formed at the sides. The mesenchyma seems to 

 be formed by special blastomeres or by cells which migrate 

 from the other layers into the segmentation cavity, thus filling 

 it. The gastral layer outpockets into the mesenchyma, estab- 

 lishing connection with the ingrowing dermal layer, thus form- 

 ing the incurrent canals (see Fig. 78). In most species the 

 process is much more complex than that described here. 



214. Classification. 



The divisions of the group Porifera are made on the basis of the differences in 

 the skeleton. Two principal groups may be recognized, as follows: 



I. Calcarea. — Sponges in which the skeleton is composed of calcareous spicules. 

 Laboratory type, — Grantia. 



II. Non-Cakarea. — Sponges with glassy (siliceous) spicules, or with homy 

 (spongin) fibres, or with merely a gelatinous mesenchyma. Laboratory types: — 

 the Venus Flower-basket; the fresh- water sponge; the commercial sponge. 



215. Ecology. — Sponges are chiefly marine animals, and 

 flotirish in all the seas and at any depth. The larger homy 

 sponges, of which the bath sponge is the skeleton, are found in 

 the warmer seas and in relatively shallow water. The best 

 ones are found in the Mediterranean, in the South Indian Ocean, 

 and about Florida and the West Indies. By reason of their 

 budding and branching, the sponges form immense colonies 

 or beds, and many other forms of life associate with them in 

 varying degrees in intimacy. Worms, Crustacea, and the 

 larvae of many forms play hide-and-seek and burrow in the 

 thickets of growth produced by sponges. In several species of 

 crabs their shells may be covered by sponges, thus becoming 

 less conspicuous. In other cases the sponges may protect the 



