It might naturally be supposed that a crea ture so low in the scale of organization would be compelled to subsist upon the simplest pos sible aliment, yet, strange to say, this little polype is carnivorous in its propensities, and is moreover gifted with such terrible powers of destruction, that animals far larger, stronger, and more active than itself fall a prey to its voracity : the Entomostracons Crustaceans, the larvm of insects, and minute Annelidans, constitute its ordinary diet, and vainly endea vour to escape from its clutches. Whilst watching for prey, the Hydra remains per fectly at rest, suspended by its tail, and keep ing its oral tentacula widely spread out in different directions, nor has it generally to wait long before some of the multitudinous animals that crowd the water in its vicinity impinge against its outspread lines, when im mediately, as if the wand of an enchanter had been laid upon it, the career of its victim is arrested; though apparently only touched, not seized, it immediately sinks motionless as though paralysed by the contact, and only after some time recovers its former vivacity. What is the benumbing power possessed by the tentacula of the Hydra it is difficult to conjecture; some writers attribute it to a tor pifying secretion ; others surmise the dis charge of an electric shock ; but whatever be its nature, its effects are sufficiently potent to prevent the escape of the animal subjected to its influences.
No sooner is the prey thus stricken motion. less than the tentacle against which it im pinged begins slowly to contract and drag it towards the orifice of the mouth. It would seem that the slightest effort on the part of the animal seized would be sufficient to tear off' the almost invisible gelatinous arm of the polype, yet not more surely does the angler land a trout by means of his sdken line, than the Hydra succeeds by its tenacious hold in securing its victim ; tentacle after tentacle is brought to bear upon it, and slowly it is ap proximated to the opening of the stomach of the polype in which it is about to be engulphed.
When lodged in the stomach of its de vourer, so thin and diaphanous is the distended bag of the Hydra's body, that the animal swal lowed is still distinctly visible, and the micro scopical observer would scarcely suspect that the pellucid film which covers it was capab'e of producing much effect upon its substance. Gradually, however, the swallowed prey begins fluid ; and, to an ordinary observer, no fibres of any kind are distinguishable in any part of to lose its distinctness of outline, and its parts become dim and confused, for the process of digestion has begun, and speedily all that is digestible is dissolved, nothing being left but the hard shell and other intractable portions, which are at length expelled from the digestive sac through the same opening by which they were admitted.
From the transparency of the Hydra, Trem bley thought to be able to ascertain the man. ner in which the digested nutriment became appropriated, and observing that the polypes became coloured in accordance with the kind of food upon which they lived, proceeded to feed them with the red larvx of certain insects, in hopes of seeing how the colouring matter became diffused through their bodies, and in this he was partially successful ; the result of his experiments proving that it was through the medium of the granules floating in the semifluid transparent substance of the Hydra that the diffusion of the coloured particles was accomplished, the granules themselves assum ing the tint of the coloured food, while the gelatinous matter in which they were sus pended remained colourless, Another remarkable fact observed by Trem bley was, that the digestive powers of the Hydra had no influence over the tissues of its own body, for frequently he observed that the long-armed species swallowed their own ten tacula along with their food, the former re maining quite intact while the latter was in process of solution, and on one occasion when two Hydrm had both of them seized on the same prey, and were cont-efiding for the possession of it, one of them decided the con test by swallowing the subject of dispute and his rival into the bargain. Naturally supposing
that the death of the swallowed polype would be the result of such an apparently tragical termination to the dispute, Trembley was not a little surprised to see the successful polype disgorge his antagonist safe and uninjured along with the egestamenta of the meal, and to all appearance none the worse for its tem porary incarceration.
If a Hydra be divided transversely, by means of a knife or a pair of scissors, both halves not only survive, but in the course of a short time each moiety reproduces the portion of which it has been deprived, the hinder extremity developing a new set of tentacula, and the an terior portion acquiring a sucker to replace that which was lost ; nay, it has been proved that even when divided into several fragments, each piece retains its vitality, and in process of time regains all the characters of a perfect individual, just as the cutting of a plant speedily puts forth roots and leaves similar to those of the original stock from which it was taken.
Not less wonderful than their capability of recovering lost parts after mutilation are the powers which they possess of multiplying their species by various modes of generation. The most usual manner in which they produce off spring is by gemmation, the nature of which, owing to the transparency of their bodies, they are admirably adapted to elucidate. The process by which this kind of reproduction is effected in the case of the Hydra is as follows. After keeping one of these polypes for a few hours well provided with food, a little bud or gemnia is seen to sprout from some portion of the surface of its body, which at first seems to be a shapeless excrescence, but in the course of a short time assumes the shape of the parent animal by developing tentacula from around the oral orifice, which gradually becomes more and more distinct. For some time the newly formed polype remains attached by the little pedicle at its tail to the body of its parent, with which it seems to enjoy a sort of com munity of existence, the food caught and di gested by the one passing freely through a little aperture in the caudal extremity of the young polype from one to the other. At last, when the growth of the off-sprout is com pleted, it detaches itself, and assumes an in dependent existence ; yet sometimes even before its separation is accomplished the bud of a third generation may be observed ap pended to the side of its body ready to under go the same process of development.