Hydra. 
Z. HYDROIDA. 
99 
The Hydrse are found in fresh and, perhaps, also in salt waters, but 
the former species only have been examined with care, and are the 
objects of the following- remarks. They prefer slowly running or al- 
most still water, and fasten to the leaves and stalks of submerged 
plants by their base, which seems to act as a sucker. The body is 
exceedingly contractile, and hence liable to many changes of form : 
when contracted it is like a tubercle, a minute top or button, and 
when extended it becomes a narrow cylinder, being ten or twelve times 
longer at one time than at another, the tentacula suffering changes in 
their length and diameter equal to those of the body. “ It can lengthen 
out or shorten its arms, without extending or contracting its body ; 
and can do the same by the body, without altering the length of its 
arms : both, however, are usually moved together, at the same time 
and in the same direction.”- — The whole creature is apparently homo- 
geneous, composed of minute pellucid grains cohering by means of a 
transparent jelly, for even with a high magnifier no defined organiza- 
tion of vessels and fibres can be detected. On the point opposite the 
base, and in the centre of the tentacula, we observe an aperture or 
mouth which leads into a wider cavity excavated as it were in the 
midst of the jelly,* and from which a narrow canal is continued down 
to the sucker. When contracted, and also when fully extended, the 
body appears smooth and even, but “ in its middle degree of exten- 
sion,” the sides seem to be minutely crenulated, an effect probably of 
a wrinkling of the surface, although from this appearance Baker has 
concluded that the Hydra is annulose, or made up of a number of 
rings capable of being folded together or evolved, and hence, in some 
measure, its extraordinary ability of extending and contracting its 
parts.f That this view of the Hydra’s structure is erroneous, Trem- 
bley has proved ; J and the explanation it afforded of the animal’s con- 
tractility was obviously unsatisfactory, for it was never pretended that 
* Pallas denies this. “ Ab alimento recepto cavatce, inquam, haud enim Hy- 
dras corpus naturaliter intestini instar cavum crediderim. Totum solidum et 
medullare, pro admoto alimento, cerse instar, digitum admittentis, cavari concipio 
parenchyma et alimentis insinuatis sese circumfundere. Qui alias per longi- 
tudinem dissecta Hydra, illico qualibet portione deglutire, et cavo clauso alimen- 
ta condere posset ? quod tamen observare rarum non est.” Elench. Zooph. 27, 
28. — For a view of the Hydra’s stomach see Tremb. Mem. pi. 4, fig. 7, copied 
by Roget in his Bridgew. Treat, ii. 74, fig. 241. 
*f- “ The outward coat is white like the arms, and made up of minute annuli 
or ringlets, that double in the midst, and can, occasionally, be folded close to- 
gether, in the manner of a paper lan thorn.” — Hist, of the Polype, 25. 
t Mem. 27. 
