30 THE FRESH- WATER POLYP 



hollow and coiled filament, and from its inner surface project the barbs. The 

 nematocyst, which contains a fluid, lies in a special interstitial cell known as 

 the cnidoblast. The cnidoblast contains a hollow sac, and in this sac the 

 nematocyst lies. The free end of the cnidoblast is produced into a delicate 

 pointed process, the cnidocil or " trigger-hair." The sUghtest touch of the 

 cnidocil causes sudden contraction of the cnidoblast, and the pressure (possibly 



Fig. 11. — A. Transverse Section of Hydra viridie (greatly enlarged) ; B. Small part of the body- 

 wall highly magnified ; C. Two isolated ectoderm cells ; D. Cnidoblast, before the extrusion 

 of the nematocyst thread ; E. Cnidoblast, after the extrusion of the nematocyst thread. 

 C.-E. after Parker and Haswell. 



through contact with the water) causes the instant eversion and extrusion of 

 the nematocyst neck and thread. There are small nerve cells connected 

 with the cnidoblasts. 



{b) The endoderm. 



This layer is made up of large endoderm cells lining the coelenteron ; these 

 are similar in shape to the ectoderm cells, and each has a nucleus, a large vacuole, 

 and also either pseudopodia or two flagella which project into the coelenteron. 

 The endoderm cells also have crosspiece muscular processes, and are therefore 

 myo-epithelial cells. The muscular processes run circularly, and can be 

 properly seen only in longitudinal sections. Some of the endoderm cells near 

 the mouth are specialised as gland cells, which secrete a digestive ferment. 

 There are also gland cells on the "foot " or base of Hydra, and their sticky 

 secretion probably enables it to attach itself firmly to surfaces. The green 



