Metazoa Obelia. 



22$ 



FIG. 115. TRANSVERSE SECTION OF 

 THE WALL OF Hydra. 



contractile, and are composed of two layers an outer ecto- 

 derm and inner endoderm. The cavity, or enteron, opens 

 to the exterior by the terminal mouth, and is continuous 

 throughout the body and into the tentacles. The ectoderm 

 is composed of large pyramidal cells, having the pointed 

 ends, which are internal, pulled out into long processes ; 

 these processes from the ectoderm cells form a tolerably 

 complete layer beneath 

 the ectoderm termed the 

 neuro - muscular layer. 

 The ectoderm cells are 

 nucleated and granular, 

 and contain, in addition, 

 pear - shaped bodies, 

 known as nematocysts. 

 The nematocysts are 

 organs of offence and de- 

 fence. Each consists of 

 a small sac in the interior 

 of which is coiled a very 

 long barbed thread, lying 

 bathed in what is no 

 doubt a poisonous fluid. 

 The animal can at will 

 evert the thread with great 

 rapidity, with the effect of 

 stupefying or poisoning 

 the prey it strikes. Nema- 

 tocysts occur in the endoderm also. The tentacles of the 

 alimentary and tentacular zooids of Obelia and other genera 

 are plentifully supplied with nematocysts. Numerous young 

 cells lie in nests round the pointed inner ends of the ecto- 

 derm cells (fig. 114), ready to take the place of the ecto- 

 derm cells when these are shed, as they frequently are when 

 a nematocyst is fired. 



The endoderm differs considerably in character from 



ectoderm ; 6, endoderm ; c, neuromuscular 

 layer ; d, basemem membrane ; <?, cilium on 

 an en odeTn cell ; _/, amoeboid endodermal 

 cell ; , ectoderm cell with young cells at 

 iis base ; Ji, /', nematocyst. 



