PORIFERA SPONGES. 



agrees generally with a simple Ccelenterate, such as Hydra, 

 but differs from it in the absence of tentacles and stinging 

 cells, and in the greater development of the mesoglcea. 



More Complicated Forms. 



But a description of a simple sponge like Ascetta conveys 

 little idea of the structure of a complex form such as the 

 bath sponge (Euspongia). Let us consider the origin of 

 complications: 



(a) Sponges long regarded as plants are plant-like in 

 being sedentary and passive. They 



seem also to feed easily and well. 

 Like plants, they form buds, the out- 

 come of surplus nourishment. These 

 buds, like the suckers of a rose bush, 

 often acquire some apparent inde- 

 pendence, and the sponge looks like 

 many vases, not like one. More- 

 over, as they grow these buds may 

 fuse, like the branches of a tree tied 

 closely together. Thus the structure 

 becomes more intricate. 



(b) In the simple sponge the 

 gastric cavity of the vase is com- 

 pletely lined by the collared endo- 

 derm cells (Ascon type). But the 

 endoderm may grow out into radial 

 chambers, and the walls of these 

 may also be folded into side aisles 

 (Sycon type). The outgrowing of 

 the endoderm into the mesoglcea 



may be continued even further, and the cells may become 

 pavement-like except in minute flagellate chambers, where 

 the characteristic collared type is retained (Leucon type). 

 (See Fig. 33.) 



[Speculatively it may be suggested that the characteristic 

 folding or outgrowth of the endoderm is necessitated by the 

 fact that the endoderm cells are better nourished and 

 multiply more rapidly than those of the ectoderm, which 

 thus fails to keep pace with the inner layer.] 



(c) By infoldings of the skin ectoderm and a subjacent 



FIG. 32. Section of a 

 Sponge. (After F. E. 

 SCHULZE.) 



Showing inhalent canals, 

 flagellate chambers, a gastrula 

 forming in the mesoglcea, &c. 



