152 AN INTRODUCTION TO ZOOLOGY 



the cell is composed of a fairly clear vacuolated cytoplasm, in which 

 are a few coarse granules and a network of delicate fibres, continued 

 through the apex of the cell into the myofibrils of the muscular 

 processes. A large spherical vesicular nucleus with well-marked 

 chromatin threads and one or two distinct nucleoli is situated near 

 the middle of the cell. In the basal disc the ectoderm cells are 

 slightly modified, are columnar in shape, and fitting close together 

 leave no interstices. They are very granular, containing minute 



droplets of a sticky secre- 

 tion, which, when passed 

 out, enables the foot to 

 adhere to the substratum. 

 These cells are, therefore, 

 not merely musculo - 

 epithelial, but also gland- 

 FIG. 49. Epithelio-muscular cells of Hydra. u l ar m character. The 

 M.,myoneme fibrill* ; N., nucleus. CCtodermal Cells in the 



tentacles also are a little 



flatter than in the body wall, and have very markedly developed 

 muscular processes. They are so closely set together that there are 

 no spaces between their inner ends. 



The remaining cells of the ectoderm, termed from their position 

 the interstitial cells or sub-epithelial cells, are small and round and 

 tightly packed together. They form as it were a reserve of im- 

 specialised cells, from which other cells of the ectoderm can be 

 replaced, and they give rise to four very distinct types. 



i. The most numerous and striking cells arising from the inter- 

 stitial ones are the wonderful structures known as the " nettle cells," 

 or cnidoblasts, in each of which is enclosed a complicated highly- 

 refractive capsule, the " stinging capsule/' or the nematocyst. 

 These capsules contained in their parent cells are to be found 

 distributed generally in the ectoderm with the exception of the basal 

 disc. In the tentacles they are particularly numerous, being 

 arranged in groups, the so-called nematocyst batteries, which give 

 the tentacles a knobby appearance. The cnidoblast is not found free 

 in the ectoderm, but completely embedded in one of the musculo- 

 epithelial cells a cell within a cell, recalling in some respects a 

 parasite. 



The cnidoblast consists of a thin layer of protoplasm enclosing 

 the capsule, and in it is situated a nucleus. From the outermost 

 part of the cell wall comes off a short slender bristle-like projection, 

 the cnidocil or trigger process. It pierces the musculo-epithelial 

 cell and cuticle and sticks out beyond the body wall. It recalls the 

 process of a sense cell, and, like it, is receptive and capable of 



