62 



SYSTEMATIC ZOOLOGY 



membrane, and come in contact with one another at the 

 mouth. Below the mouth there are one or two rings of tenta- 

 cles. The coelenteron is a simple cavity whose walls are rela- 

 tively smooth and never thrown into folds or ridges. These 

 polyps are frequently very delicately colored and live singly 



Fig. 45. Xematocysts of Hydra. A, undischarged: B, discharged; C, nerve 

 supply. cnb, cnidoblast; cnc, cnidocil; nit, nucleus; ntc, nematoi 

 nv.c, nerve-cells. (After Schneider, from Parker and Haswell's Manual.) 



or in colonies (Fig. 47). The latter are often large and very 

 beautifully branched so that they look much like plants. All 

 the polyps in a colony are connected with one another by layers 

 of cells, called the ccenosarc ; sometimes the colonies are not 

 branched, but the polyps arise side by side from the ccenosarc, 

 which extends over a considerable area (Fig. 47). These colo- 



