60 ZOOLOGY DIEECTIONS 



which lies next to the mesogiea. These are muscle threads cut 

 in cross section, 



(b) The netting cells or cnidohlasts are peculiar in that 

 they are included within the protoplasm of the epithelio-muscu- 

 lar cells. The nematocyst may be distinguished as a clear blad- 

 der-like structure which frequently encloses a solid elongated 

 body. This last named structure is the series of barbs which be- 

 come evident on one type of netting cells when they are dis- 

 charged. A small nucleus is often present just outside the wall 

 of the nematocyst. The nucleus with the protoplasm immedi- 

 ately around the nematocyst constitutes the cnidohlast, or cell 

 which produces the nematocyst. In a nettling cell which lies at 

 the outermost surface of the ectoderm a small pointed projection 

 called the cnidocil is frequently observable extending beyond 

 the general surface of the body. The cnidocil used to be called 

 the "trigger" on the supposition that it controls the explosion 

 of the nematocyst. 



(c) Small wedge-shaped cells called interstitial cells are 

 frequently found between the bases of the epithelio-muscular 

 cells. It is from these cells that the nettling cells are formed. 

 When fully formed the nettling cells migrate through the proto- 

 plasm of the epithelio-muscular cells until they come to lie at 

 the surface of the body where they are in a position to function. 



(d) The entoderm cells are much larger than ectoderm 

 cells. The protoplasm of these cells is usually fiUed with rounded 

 masses of stored food material which has been taken directly 

 from the coelenteric cavity. These masses are frequently so 

 numerous as to obscure the nucleus. 



Make a drawing of a part of the wall of the body as seen 

 under high power. Show the structure very carefully in one or 

 two cells of each kind. 



In your 200 mm. drawing of the entire animal, show in out- 

 line the location and distribution of coelentric cavity, ectoderm, 

 and entoderm. 



