ECTODERM 



93 



which the thread is driven (Fig. 60). In piercing the chitin of 

 arthropods, upon which the Hydra feeds, they are assisted 

 by the corrosive action of a fluid which they contain, either 

 in the hollow of the tucked-in thread or in that of the sac. This 

 fluid also temporarily numbs the prey, but the main function 

 of the mematocysts is not to kill but to hold the prey until it 

 is swallowed. In this the spiral kind possibly assist by coiling 

 round bristles upon the body of the prey, while the other nemato- 



Chibin 

 Terminal bubc 



oLubion oF chibin 

 by capsular 

 conbenbs 



PcrForabion dus 

 bo mechanical 

 action oF sty Ubs 



Fig. 60. — Nematocyst of Hydra with the tube perforating the chitin of an 

 arthropod. — From Yapp, after Toppe. Yapp, An Introduction to Animal 

 Physiology, 1939. Clarendon Press, Oxford. 



cysts are perhaps of use in attaching the tentacles of the hydra, 

 either to its prey or to other objects, by the stickiness of their 

 threads. The cnidoblasts arise from the interstitial cells by 

 the formation of a vacuole and its gradual modification into a 

 nematocyst. They are formed in the upper region of the cylinder 

 and migrate thence to various parts of the body, where they take 

 up their position in the outer layer. The germ cells also arise 

 in the ectoderm from the interstitial cells by a process which we 

 shall describe later. Lastly, among the bases of the ectoderm 

 pillars lies a mesh-work of branching nerve cells which is joined 

 by rootlets from tall, narrow sense cells that, like the cnidoblasts 



