16 



HYDBA 



They are capable of free and active movement, and 

 are used for capturing and paralysing the prey, 

 which they then, by their contraction, transfer to 

 the mouth. 



Fig. 12. — Hydra viridis. A small portion of a longitudinal section 

 through the body-wall ; x 800. (a. m. m.) 



A, one of the large ectoderm cells. B, nucleus of a large ectoderm cell. 

 C, muscular tall of a large ectoderm cell. D, an undischarged nematocyst 

 lying within its parent cell, or cnidoblast. E, cnidocil, IP, a nematocyst with 

 discharged thread. G-, Interstitial cell, £[. supporting lamella. I, endoderm 

 cell. K, vacuole of endoderm cell. L, nucleus of endoderm cell. M, chloro- 

 phyll corpuscle. "N, flagellum of endoderm cell. 



B. Structure. 

 1. Structure of the body-wall. 



Examine the body-wall more closely with the low power, 

 and note its division into layers. 



a. The ectoderm is the outermost layer of the body. 

 It is colourless, and forms about a third of the 

 entire thickness of the wall. 



