FEESH-WATEE POLYP 87 



body, including the tentacles ? How much of the 

 thickness of the body- wall does this layer form ? 

 What are the shape, relative size, color, and ar- 

 rangement of the cells in this layer? Examine 

 both extended and contracted specimens with re- 

 gard to the points last mentioned. Scattered 

 among the ectoderm cells on the tentacles, no- 

 tice the "thread cells" or cnidoblasts, in which 

 lie highly refractive capsules, the nematocysts, 

 each enclosing a spirally-coiled thread, and the 

 outer end of which terminates in a sharp spine, 

 the cnidocil. On what part of the tentacle are 

 the nematocysts most abundant? Are they regu- 

 larly arranged? Do you find any on the body 

 proper ? On the hypostome ? How do they com- 

 pare in shape and size with the ordinary ectoder- 

 mal cells? Notice that, in addition to the two 

 kinds of cells already studied, the ectoderm also 

 contains among the bases of the larger cells small 

 or interstitial cells. Compare these with the 

 others. 



Does the entoderm line the entire body-cavity ? 

 Does it extend into the tentacles ? How does it 

 compare with the ectoderm in thickness ? Are its 

 cells larger or smaller than those of the ectoderm ? 

 What is their shape ? Color ? To what is the 

 color due? Can you see a circulation of food- 

 particles in the body-cavity ? In the tentacles ? 

 Can you detect the cause of this circulation ? Do 

 any of the entodermal cells bear cilia or flagella? 

 What changes take place in the entoderm when the 

 animal contracts ? How thick is the supporting 

 layer? Can you trace it throughout the body ? Can 

 you discover whether or not it is composed of cells? 



