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. Calenterata. 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. 



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

 current pores; ip, excurrent pores; mes, mesenchyme; o, 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. 



