Natural History of British Zoophytes. 333 



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 

 lessen its diameter, either at the root, at the extremity, in the mid- 

 dle, or where it pleases : which occasions a great variety of appear- 

 ances, 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 

 tube has become too fine in its bore for the admission of water attenu- 

 ated to its smallest possible stream, — how inconceivable 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.t Its locomotion is always very slow, and the disposi- 



• Mem, 123—5; and 263. 



t '• The position in which they appear to take most delight, is^that of remain- 

 ing suspended from the surface of the water by means of the foot alone : and 



