126 PORIFER A— SPONGES. 



ship or symbiosis. Finally, sponges deserve mention as 

 factors in human civilisation. 



General Zoological Interest and Position. — Sponges have 

 this great interest, that they form the first successful class 

 of Metazoa. They illustrate the beginnings of a " body " — 

 the beginnings of tissues. Along with the Ccelentera, from 

 which it is the almost unanimous opinion that they must 

 be held distinct, they differ markedly from the triploblastic, 

 Ccelomate Metazoa, which do not retain the radial symmetry 

 of the gastrula. 



Their origin is wrapped in obscurity, though there is 

 much to be said for the view that they are the non-pro- 

 gressive descendants of primitive gastrula-like ancestors of 

 sluggish constitution. It does not seem likely that they 

 have led on to anything higher, they rather represent a 

 by-road in Metazoan evolution. 



