84 



ORGANISMS OF TISSUES 



called nematoblasts or cnidoblasts, thus perform functions of 

 offence and defence. 



Each stinging cell possesses a sensory hair or " trigger" 

 called a cnidocil at the free end, and a thread-holding capsule 

 the nematocyst within. The thread is formed from a cell growth 

 which is spirally wound in the capsule, while capsule and its 

 contents are all formed by differentiation of a single nucleated 

 cell of the ectoderm. During growth of the capsule and 



FIG. 36. Diagrammatic figure of the cells from a small portion of the body wall 

 of Hydra viridis; the ectodermal cells with nematocysts below (one with pro- 

 truded thread), and the large vacuolated endoderm cells above. The symbiotic 

 algae are grouped near the supporting lamella at the bases of the endoderm cells. 

 (From Marshall and Hurst.) 



thread the young nettle cells lie near the supporting lamella 

 in the region of the mouth, but they migrate during the period 

 of their formation and finally come to lie on the surface of the 

 ectoderm around the mouth or on the tentacles and body, 

 the cnidocils ultimately projecting slightly beyond the surface 

 of the body in the surrounding medium (Fig. 36). 



3 . Nerve Cells. The nerve cells of Hydra and the coelenterates 

 represent the special differentiation of cells for the performance 



