100 Z. HYDKOIDA 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, but they seem to originate a little 

 under the lip, for the mouth is often protruded Hke 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, foi'ra 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-iish.* Trerabley 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 blnnt, 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 cyhnder or bowel, and that the tentacula are tubular and 

 have a free communication with its cavity ;J 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 outvvards 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 papilke 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. 



t Mem. 108. 



t Ibid. 123—5; and 26-3. 



