16 



HYDEA 



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. 5. — Hydia yiridis. A small portion of a longitudinal section 

 through the body -wall; x800. (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 nematooyst 

 lying within its parent cell. B, onidocil. F, a nematooyst with discharged 

 thr^. G, interstitial oell. H, mesoderm lamella. I, endoderm cell. K, 

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

 corpuscle. M", 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 di/oision 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. 



