100 
Z. HYDROIDA 
Hydra. 
such an anatomy could be detected in the tentacula, which, however, 
are equally or more contractile. These organs encircle the mouth 
and radiate in a star-like fashion, hut they seem to originate a little 
under the lip, for the mouth is often protruded like a kind of small 
snout : they are cylindrical, linear or very slightly tapered, hollow 
and roughened, at short and regular intervals, with whorls of tuber- 
cles which, under the microscope, form a very beautiful and interest- 
ing object ; and I have thought, when viewing them, that every lit- 
tle tubercle might be a cup or sucker similar to those which garnish 
the arms of the cuttle-fish.* Trembley has shewn us that this is a 
deception, and that there is really no exactness in the comparison. t 
The tentacula are amazingly extensible, from a line or less to one or, 
as in H. fusca, to more than eight inches ; and “ another extraordi- 
nary circumstance is, that a polype can extend an arm in any part of 
its whole length, without doing so throughout, and can swell or les- 
sen its diameter, either at the root, at the extremity, in the middle, 
or where it pleases : which occasions a great variety of appearances, 
making it sometimes terminate with a sharp point, and at other 
times blunt, knobbed, and thickest at the end, in the figure of a bob- 
bin.” We naturally enquire how this wonderful extension is made, — 
by what power a part without muscularity is drawn out until it ex- 
ceeds by twenty or even by forty times the original length ? The 
dissections of Trembley have proved beyond any doubt that the body 
is a hollow cylinder or bowel, and that the tentacula are tubular and 
have a free communication with its cavity and in this structure, 
combined with the loose granular composition of the animal, we find 
an answer to the question. Water flows, let us say by suction, into 
the stomach through the oral aperture, whence it is forced by the 
vis a tergo, or drawn by capillary attraction, into the canals of the 
tentacula, and its current outwards is sufficient to push before it the 
soft yielding material of which they are composed, until at last the 
resistance of the living parts suffices to arrest the tiny flood, or the 
* Pallas has the same suggestion. Elench. 26. See also Roget’s Bridgew. 
Treat, i. 182.— Baker says that “ two or three pretty long hairs” issue from each 
of the papillae or tubercles, p. 36 ; and Trembley has figured a short hair issu- 
ing from some of them, Mem. 62, pi. 5, fig. 3. This appearance of hairs is, I 
presume, produced by the glutinous secretion from them being drawn out into 
fine lines and drying on the glass. The tentacula probably adhere to foreign 
bodies principally by means of a mucous excretion, and being as it were en- 
grained into the microscopic interstices of the body to which they are applied — 
Tremb. Mem. 46. 
f Mem. 108. 
X Ibid. 123—5; and 263. 
