16 



HYDKA 



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. 



FHJ. 5. 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 tail of a large ectoderm cell. D, an undischarged nematocyst 

 lying within its parent cell. E, cnidocil. F, a nematocyst with discharged 

 thread. Q-, interstitial cell. H, mesoderm lamella. I, endoderm cell. K, 

 vacuole of endoderm cell. L, nucleus of endoderm cell. M, chlorophyll 

 corpuscle. N", flagellum of endoderm cell. 



B. Structure. 

 1. Structure of the body-wall. 



Examine the body -watt 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. 



