Ectoderm 



Endoderm 



84 ESSENTIALS OF ZOOLOGY 



thread. The cell itself is otherwise simple ; it has a nucleus, 

 and the cytoplasm projects at one point of the margin as a 

 process termed the cnidocil. The cnidocil is irritable, and 

 when touched it is able to set up a compression aided probably 

 by an influx of water resulting in the expulsion of the thread, 

 which in the process is turned outside in. The process may 



be readily observed by 

 adding a little dilute 

 acetic acid to the water 

 containing the Hydra. 

 If the effect be watched 

 under the microscope it 

 will be found that there 

 are two kinds of cnido- 

 blasts. One has a long 

 thread which is shot out 

 with sufficient force to 

 be penetrating, and the 

 thread is armed by barbs 

 at the base. This is the 

 poisonous or stinging cell 

 proper, and its function 

 is not only to help in 

 securing the prey but to 

 paralyse it. The other 

 has a shorter and thicker 



Cnidocil 



Capsule 



FIG. 15. Section of body wall of Hydra. 

 In the ectoderm the ectodermal cells 

 often present vacuoles, and are sepa- 

 rated at the base by interstitial cells. 

 This region is occupied also by nerve 

 cells. The endoderm cells are occupied 

 by numerous zoochlorellae. Below are 

 shown the two types of stinging cells. 



thread which even in the 

 free condition assumes a 

 spiral disposition, and it 

 is unarmed. Its purpose is to wrap itself round the prey, or a 

 part of it, and thus to prevent its escape. 



Endoderm. The endoderm is practically colourless in 

 H. vulgaris, it is brown in H. fusca, and green in H. viridis. 

 The column of endoderm thus stands out prominently in the 

 coloured species in contrast with the colourless ectoderm. 

 Sections will show that the endoderm cells are arranged as an 

 epithelium bounding the gastric cavity. They are larger than 

 the epithelial cells of the ectoderm, and are either flagellate or 

 amoeboid on the inner or free border. The brown colour of 



