HYDRAID^ I HYDRA. 129 



extensibility of these organs, from a line to one, or, as in H. fusca, 

 to upwards of eight inches ; and to produce this degree of elonga- 

 tion, it seems necessary to have superadded the propulsive agency of 

 a fluid. 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 attenuated 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 imper- 

 ceptible slowness on the base, or by stretching out the body and 

 tentacula 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 retro- 

 grade. Its ordinary position seems to be pendant, or nearly hori- 

 zontal, 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 sometimes with the tentacula downwards ; and again it will 

 lay itself along horizontally.* Its locomotion is always very slow, 



* " The position in which they appear to take most delight is that of remaining 

 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 dragged 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 sur- 

 face. 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 hydro- 

 static power will be destroyed, and the animal will immediately sink to the bottom." 

 Roget, Bridgew. Treat, i. 179. This passage is nearly a literal translation from 

 Trembley's Hist, des Polypes, pp. 37-8. 



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