86 



ORGANISMS OF TISSUES 



are sometimes called nematoblasts or cnidoblasts, thus perform 

 functions of offence and defence. 



Each stinging cell possesses a sensory hair or " trigger," 

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

 the nematocystj 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 thread the young nettle cells first lie near the supporting 



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

 of Hydra mridis; 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.) 



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

 the period of their formation, and finally come to lie on the sur- 

 face 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 coelenter- 

 ates represent the special differentiation of cells for the per- 



