STRUCTURE OF BODY-WALL. 



21 



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. 13. 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. Bi nucleus of a large ectoderm cell. 

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

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

 discharged thread. G, interstitial cell. H , supporting lamella or mesoglcea. I, 

 endoderm cell. K, vacuole of endoderm cell, l, nucleus of endoderm cell. 

 M, green-cells. 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. 



b. The endoderm is the inner layer, lining the diges- 



tive cavity, and forming about two-thirds of the 



