2 24 COMPARATIVE ANATOMY 



into the digestive cavity. The distal moiety of the body 

 is often swelled up by the presence of one or more water- 

 fleas, which have been swallowed and are undergoing digestion, 

 but they are seldom to be seen in the proximal moiety. 

 Crustaceans have hard chitinous exoskeletons which cannot 

 be digested. These and other indigestible matter are afterwards 

 expelled by the mouth. 



The general structure of Hydra can be studied by subjecting 

 the animal to slight pressure under a cover-glass and examining 

 it under a moderate power of the microscope ; but in order to 

 study details one must have recourse to, very thin longitudinal 

 and transverse sections and to the method of maceration. By 

 the first method of examination it can be seen that the body 

 is a simple sac, having a single spacious internal cavity, the 

 gastrovascular cavity, whose walls are formed of two layers of 

 cells separated by a fine membrane. The external layer of 

 cells, forming the outside skin of the body and tentacles, is 

 colourless, not very thick, and studded, especially on the distal 

 moiety of the body and on the tentacles, with numerous small 

 ovoid capsules, the nematocysts. This outer layer is known 

 as the ectoderm. The inner layer, or endoderm, is notably 

 thicker, and is full of granules and small brown corpuscles 

 which render it opaque. The colour both of H. fiisca and H. 

 viridis is due to the brown or green corpuscles contained in 

 the endoderm cells, and the distinction between the colourless 

 ectoderm and the endoderm stuffed full of green corpuscles is 

 particularly well marked in the latter species. 



Between ectoderm and endoderm is a very thin layer of 

 colourless gelatinoid material, distinguishable as a transparent 

 line in optical section. This is the jelly, or mesoglaea, very 

 scantily developed in Hydra, but attaining great thickness and 

 forming the greater part of the bulk of the body in many 

 Coelenterates. It is not formed of cells as the ectoderm and 

 endoderm are, but is formed as a secretion of (probably) both' 

 of these layers. 



The general structure of the tentacles is the same as that of 

 the body. They are simply hollow processes of the latter, 

 each containing a prolongation of the gastrovascular cavity, 

 bounded by the two cellular layers ectoderm and endoderm, 

 with the mesogloea between. When retracted a tentacle is 

 thick and bluntly finger-shaped, its surface thrown into a 



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