244 



COMPARATIVE ANATOMY 



is to catch, swallow, and digest prey, and, since they are all 

 in communication, there is a community of nutrition among 



Fig. 52. 



Longitudinal section through a hydranth of Obelia. 

 geniculata ; nt, mouth ; hy, placed in the cavity of 

 the hypostome ; gv, gastrovascular cavity ; ec, 

 ectoderm ; en, endoderm ; ing, mesogloea ; nc, nemato- 

 cysts on the tentacles ; en', large vacuolated endoderm 

 of the solid tentacle ; ht, hydrotheca. 



the members of the colony. But a hydranth has nothing to 

 do with reproduction, except that it gives rise to a bud 

 in prolongation of the colony. (For it must be understood 



