The body wall is composed of two membranes, an outer and an 

 inner. The former is called the Ectoderm ; making up the entire 

 outer surface, it is colourless and forms about one-third of the entire 

 thickness of the body wall ; and the latter the Endoderm, or iuner 

 skin lining the interior of the body, which consists of a simple 

 stomach cavity from which effete matters are ejected by means of 

 the mouth. This makes up about two-thirds of the entire 

 thickness of the body wall, and is coloured green or brown 

 according to the species of Hydra examined. 



Between the Ectoderm and Endoderm is a very thin gelatinous 

 layer which is hardly visible with a low power. The Hyposlome, or 

 oval cone, is the part of the body above the tentacles, which is 

 ■conical when protuded, the mouth being situated at its apex. The 

 tentacles, which are hollow cavities continuous with the gastric 

 •cavity are arranged in a simple whorl around the base of the 

 hypostome ; when fully extended they are two or three times the 

 length of the body ; when entirely contracted they are scarcely 

 visible. They exercise power over living prey much greater than 

 would be expected from their size. On close inspection the 

 tentacles are seen to be covered with minute sacs whose outer thin 

 walls are easily burst by pressure, and when this occurs a long 

 whiplash-hke filament which lay coiled within the cell is suddenly 

 projected, thus rendering the tentacle a formidable organ for 

 iseizing prey, the action being either mechanical or by virtue of 

 the possession of some poisonous fluid which benumbs the prey, so 

 •that it falls a ready victim. This stinging process is denied by 

 Lewes in his " Seaside Studies." 



I will now give, in a slightly different form, his words on this 

 point. He begins thus: "I saw the tiny water fleas drop 



• apparently lifeless to the bottom of the phial, after being held for 

 some time by the tentacles of the Hydra, and, intently watching 

 them, I saw Uiem at last swim away again as lively as before. I 

 removed a hydra from the phial, in a little water and placing it 



• on a slip of glass, allowed it to settle and expand there for two 

 hours, when 1 added several water fleas to the little pond and 



^patiently watched them swimming to and fro. Repeatedly they 

 touched tiie tentacles in their course, bur, wera not liurt, were not 

 even arrested. At length one was caught and held for some 

 seconds ; it then fell to the bottom and remained motionless 

 for at least two minutes, after which it started up and was 

 off as if its course had never been arrested. This certainly 

 had very much the appearance of a case of slight paralysis ; 

 the animal seemed arrested by some benumbing influence 

 which for two minutes rendered it powerless. At the ex- 

 piration of that time it seemed to have sufficiently recovered 



.itself to swim away. If observation alone sufficed, in questions 



