HYDRA 49 



apparatus is like the inturned finger of a glove which can 

 be blown out by pressure from the inside. The mechan- 

 ism is simple. The cnidocil or trigger-hair is touched by 

 some animal, an impulse is conveyed to the delicate fibres 

 interspersed among the cells (nerve-cells) which stimulate 

 the cnidoblast cell, whereupon there is a contraction of 

 the contents and, the cnidoblast being compressed, the 

 inverted whip-like process turns wrong side out and im- 

 pales the animal on its points or barbs. 



TECHNICAL NOTE. The teacher should be provided with micro- 

 scopical sections, both transverse and longitudinal, of the Hydra 

 stained in some good general stain (hsematoxylin or borax carmine). 

 If the teacher has no means of making such preparations, they may 

 be procured from dispensers of microscopical supplies. 



From the cross-section of the Hydra make out the 

 general structure of the body. Note that it is a hollow 

 cylinder consisting of two well-defined layers of cells, an 

 outside ectoderm layer and an inner endoderm layer. 

 Between these two is yet another thin non-cellular layer 

 called the mesogloea. 



Thus it will be seen that Hydra is made up of two 

 layers of cells, the outer ectoderm or skin, which is 

 specialized to perform the office of capturing prey as well 

 as that of protection, and the inner endoderm, whch sur- 

 rounds the digestive cavity and performs the function of 

 digestion. The endoderm lines the body-cavity, particles 

 taken in as food being digested by certain digestive cells 

 which thrust out amoeboid processes and ingest particles 

 of food. Other cells in the endoderm have long flagellate 

 processes which vibrate back and forth in the digestive 

 cavity, thereby creating currents in the water containing 

 food-particles. 



Note, in a cross-section, that there are small ovoid or 

 cuboid cells at the bases of the large ectoderm cells. 

 These are the interstitial cells. Some of the interstitial 



