244 THE MICROSCOPICAL NEWS. 
plant. The cavity of the body alone represents a stomach, or any- 
thing that may be likened to an intestinal organ. There are no or- 
gans of circulation, respiration, or urinary secretion. When food is 
taken, the nutritive matter is quickly dissolved out and absorbed 
into the substance of the Hydra. ‘The products of digestion are 
then transmitted by imbibition from cell to cell, and the waste of 
the cells is exuded directly into the water. 
The mouth is situated at the anterior end, and when extended, 
forms a small cone. The opening of the mouth can easily be dis- 
cerned when the animal is partaking of its food, for then its mouth 
dilates according to the bulk to be swallowed, which is often larger 
than the normal size of the animal. 
The tentacles are arranged around, and just below the mouth. 
They differ in number and shape, and are hollow processes, which 
are closed at the anterior end by a small knob containing a num- 
ber of thread cells. At the base they open directly into the 
stomach cavity. On the external surface the tentacles are very 
freely covered with thread cells, and when irritated by magenta 
staining, these threads are thrust out, and are found to consist of 
three forms. 
First—Fig. 63. An oval or pear-shaped capsule with a filament 
many times its own length attached to one end, and three short 
spines at the base of the thread. 
Second—Fig. 64. Smaller thread cells, without the spines, and 
with a short thread. 
Third—Fig. 65. Cells like No. 2, but with a much longer thread. 
These threads, when not in use, are coiled up in their cells (Fig. 
66), which have thick and elastic walls, and appear slightly tinged 
with a greenish colour. It is by means of these so-called threads 
that the Hydra has the power to sting or paralyze its prey. When 
these capsules, containing the threads, are heated with a solution 
of nitrate of silver, a portion of the silver is reduced to the metallic 
state. This action may be due to formic acid, which acts ina 
similar manner. ‘These organs closely resemble in structure those 
of the Medusa, which possess an urticating power like stinging 
nettles, also arising from the presence of formic acid. 
Fig. 67 shows the tentacles in a contracted state. 
Fig. 68 shows that of expansion. Within the tentacles can be 
observed a protoplasmic fluid. When they elongate they become 
narrower and taper to the free end. At this time the contained 
fluid rushes upward, and the corpuscles (s) floating along beside 
them may also be seen. When contraction takes place, the fluid 
runs back again, the tentacles become wider and shorter ; so much 
so at times that they can hardly be seen. How the elongation of 
the body and tentacles takes place is not yet understood. 
The posterior part of the body is more or less dilated into a 
