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The body wall is composed of two membranes, an outer and an 
inner. The former is called the Zctoderm ; 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 inner 
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 Eetoderm and Endoderm is a very thin gelatinous 
layer which is hardly visible with a low power. The Hypostome, 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-like filament which lay coiled within the cell is suddenly 
projected, thus rendering the tentacle a formidable organ for 
“seizing 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 them at last swim away again as lively as before. I 
‘removed a hydra from the phial, ina little water and placing it 
on a slip of glass, allowed it to settle and expand there for two 
hours, when { added several water fleas to the little pond and 
ipatiently watched them swimming to and fro. Repeatedly they 
touched tiie tentacles in their course, but were not hurt, 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 
dtself to swim away. If observation alone sufficed, in questions 
