Hydra. 
Z. HYDROIDA. 
101 
tube has become too fine in its bore for the admission of water attenu- 
ated to its smallest possible stream, — how inconceivably slender may 
indeed be imagined, but there is no thread fine enough to equal it, 
seeing that the tentacula of Hydra fusca in tension can be compared 
to nothing grosser than the scarce visible filament of the gossamer’s 
web. 
The Hydra, though usually found attached, can nevertheless move 
from place to place, which it does either by gliding with impercep- 
tible slowness on the base, or by stretching out the body and tenta- 
cula to the utmost, fixing the latter, and then contracting the body 
towards the point of fixture, loosening at the same time its hold with 
the base ; and by reversing these actions it can retrograde. Its or- 
dinary position seems to be pendant or nearly horizontal, hanging 
from some floating weed or leaf, or stretching from its sides. In a 
glass of water the creature will crawl up the sides of the vessel to 
the surface, and hang from it, sometimes with the base, and some- 
times with the tentacula downwards ; and again it will lay itself along 
horizontally.* Its locomotion is always very slow, and the disposi- 
tion of the zoophyte is evidently sedentary ; but the contractions and 
mutations of the body itself are sufficiently vivacious, while in seiz- 
ing and mastering its prey it is surprisingly nimble ; seizing a worm, 
to use the comparison of Baker, “ with as much eagerness as a cat 
catches a mouse.” It is dull and does not expand freely in the dark, 
but enjoys light, and hence undoubtedly the reason why we generally 
find the Hydra near the surface and in shallow water. 
The Hydrse are very voracious, feeding only on living animals, fbut 
* “ The position in which they appear to take most delight, is that of re- 
maining suspended from the surface of the water by means of the foot alone : 
and this they effect in the following manner. When the flat surface of the foot 
is exposed for a short time to the air, above the surface of the water, it becomes 
dry, and in this state exerts a repulsive action on the liquid, so that when drag 
ged below the level of the surface, by the weight of the body, it still remains 
uncovered, and occupies the bottom of a cup-shaped hollow in the fluid, thereby 
receiving a degree of buoyancy, sufficient to suspend it at the surface. The 
principle is the same as that by which a dry needle is supported on water, in the 
boat-like hollow which is formed by the cohesive force of the liquid, if care be 
taken to lay the needle down very gently on the surface. If, while the Hydra is 
floating in this manner, suspended by the extremity of the foot, a drop of water 
be made to fall upon that part, so as to wet it, this hydrostatic power will be 
destroyed, and the animal will immediately sink to the bottom.” — Roget. Bridgw. 
Tr. i. 179. This passage is nearly a literal translation Iron Trembley’s Hist, 
des Polypes, p. 37-8. 
f In confinement however, Trembley found that they might be fed on min- 
ced fish, beef, mutton, or veal — Mem. 104. 
