74 



latter the dermal layer is developed. The wall is thickened 

 by growth of the middle jell y, and the radial chamber* 

 are formed by infolding. 



Flagellate Cells 



Non-flagellate 

 Central Cavity^' m-.::*: : Mar Ce 



FIG. 17. Amphiblastula larva. (After Scbulze.) 



Describe the Life- history of the Freshwater Sponge (Spongilla], 



In autumn gemmules are formed within the sponge. 

 The gemmule is a group of food-laden cells enclosed in a 

 protective envelope of capstan-like spicules made by 

 the scleroblasts. The rest of the sponge decays, but the 

 gemmules remain alive. In spring these develop into- 

 new sponges, some producing spermatozoa, others ova_ 

 From the fertilised ova a summer generation of sponges 

 is developed. These produce gemmules in autumn and 

 then perish. 



This life-history shows alternations of generations. 



PHYLUM CCELENTERATA. 



Give an Account of the General Characters of the Coelenterata* 



The body is radially symmetrical. The body- wall con- 

 sists of two layers, outer ectoderm and inner endoderm^ 

 with mesoglcea between. The central space of the body 

 is the coelenteron, and its opening is the mouth. 



The layers show differentiation into special contractile 

 and nervous tissues. Cnidoblasts or stinging cells are a- 



