230 



CLASSIFICATION OF ANIMALS 



and a middle mesoderm. In the coelenterates the mesoderm is only 

 incompletely developed and in some cases entirely wanting. Reproduc- 

 tion is generally of the sexual type though in the lower phyla asexual 

 methods often occur in addition to the sexual. 



518. Phylum II. Ccelenterata. — In the Coelenterates the 

 mesoderm is represented usually by a gelatinous matrix con- 

 taining various cellular elements which are derived either from 

 the ectoderm or entoderm. There is no body cavity or vas- 

 cular system. The gastric cavity is extended by canal-like 

 prolongations into all parts of the body. 



•^c- 



FiG. 129. — Diagram of a sponge, c, Cloaca; ch, flagellale chambers; sp, in- 

 current pores; ip, excurrent pores; mes, mesenchyme; 0, osculum; r.c, radial 

 canals. (From Galloway.) 



519. Class I. Porifera. — Sponges are all marine with the 

 exception of a single fresh-water genus, Spongilla. They are 

 always attached to some object and sometimes bore into shells 

 or calcareous rocks. They vary greatly in form; sometimes 

 covering the substratum like a thin velvety crust, sometimes 

 rising into conical, spherical, cylindrical or vase-shaped masses. 



