100 



Coelentera. 



fibres; such, cells^ which only occur among Coelentera^ are termed 

 epithelio-muscle cells.* They may, however, lose their- 

 close connection with the rest of the epithelium, and may lie 

 within this as simple muscle cells; then the non-contractile 

 portion of the protoplasm, together with the nucleus, lies as in 

 the muscle cells of many other animals on one side (the outer) of 

 the contractile fibril. Further between the true epidermal cells, or 



Fig. 59. Cells of a Coelenterate (Actinian). u, epithelio-muscle cell, h muscle cell, 

 c — d ganglion cells, e sensory cells, / nettle cell, g nematooyst witli everted head. — Partly 

 after Hertwig, partly Orig. 



between the epithelio-muscle cells, sensory cells may be present. 

 Each of these bears at its outer free end, a single delicate sensory 

 hair, whilst its inner end is produced into one or several fine threads. 

 Between the inner ends of the other cells, or entirely below these, 

 lie certain cells which from their general appearance must be re- 

 garded as ganglion cells ; from each, several delicate fibrils run 

 out in various directions, and ramify among the similar fibrils 

 of the sensory cells. In the ectoderm, there are, moreover, a number 

 of nettle cells, each of which bears a fine hair at its outer end, 

 and encloses a so-called nematocyst, a tiny bladder-like structure,, 

 containing a coiled thread, which under certain conditions, notably 

 if the animal is irritated, can be shot out with great force; within 

 the threads is an irritative fluid, which may cause a burning sensation 

 in the skin of man, more powerful in some forms than in others, 

 whilst it paralyses or kills smaller animals. Lastly, there are 

 beaker-shaped cells which secrete the mucus with which these 

 forms are so often covered (gland cells). It must be noticed, 

 further, that the cells which have been described as constituting 

 the ectoderm, are not uniformly distributed over the whole body, 

 but on the contrary, occur very unequally in different regions ; 



* Quite recently it has been stated that epithelio-nmscle cells occur also in. 

 Echinodcrma and some Ghfetopoda. 



