The formation of the reproductive gemmre may even be determined by extraneous causes; thus Trembley noticed that by snipping the side of an adult polype with the points of a fine pair of scissors, a bud would soon develope itself from the wounded part ; and this experi ment might be repeated again and again, both upon the original polype and the progeny thus made to sprout from its sides, until as many as seventeen have been obtained, all con nected with each other, and thus forming a little tree of living polypes.
Besides the gemmiparous mode of repro duction, Hydrm have been occasionally ob served to divide themselves spontaneously by transverse fissure, and thus separate into two animals, in the same way as some df the Poly gastric animalcules.
The anatoniy of the Hydra has been recently closely investigated by Corda, whose obser vations upon this subject are possessed of extreme interest.* According to this observer each tentacule of the Hydra consists essentially of a long, pellucid, and extremely delicate membranous tube (fig. 27) containing an al most fluid albuminous substance, which in certain definite localities swells into denser wart-like knots (h), arranged in a spiral line, along which are appended organs of touch (d), and also instruments of prehension (c). Si tuated within the tube, and running imme diately beneath the above-mentioned nodosities, which are arranged in a quaternian series, are situated four longitudinal bands of muscular fibres of a slightly yellow colour (e), which seem to be the extensors of the tentacles.
These extensors of the tentacula are more over united to each other by transverse cular fasciculi (f) of the same colour as selves, which Corda names adductors of the tentacles, as he considers that when fully tended the tentacle is by their action folded up like a fan. The observations of the same writer lead him to consider that there is no communication whatever between the interior of the tentacula and the cavity of the body, for the tentacle is filled up with the albuminous material in which are dispersed brown co loured granules, apparently of an oily nature.
In the wart-like nodosities which wind in a spiral course round the tentacula, Corda finds what he considers to be organs of touch (fig. 27, d, and A. 28, I, 2). Each of these consists of a delicate sacculus implanted in the wart-like excrescence (fig. 28, p), which encloses another (q), provided with thicker walls and containing in its interior a minute cavity (r). Every one of the singular bodies thus organized supports an almost impercep tible filament (s), completing the supposed tactile apparatus.
In the midst of every group of these filament-supporting vesicles was found an instrument adapted to seize prey, which its discoverer names a dart (hasta). This apparatus consists of a transparent oval sac (fig. 28, /), imbedded in the substance of the
tentacle (k), and furnished above with a small orifice (h). At the bottom of the sac is situated a saucer-shaped body (nz), upon which is placed a solid oval corpuscle (n), and this again supports a long and sharp spiculum (sagitta), composed of calcareous matter (o), capable of protrusion and retraction through the aperture h, apparently by the eversion and retroversion of the saucer-shaped bladder to which the oval basis of the dart, n (has tjfer), is appended. Whenever the Hydra would seize any animal, the darts of the tentacle become extruded, and its whole sur face is thus rendered tenacious. Yet this does not seem to be all ; it would appear that the sagittm are empoisoned, as an animal once laid hold of by the Hydra very speedily dies.
At the base of the tentacula, the open ing of the mouth is surrounded with lips capable both of inflection and protrusion. This lip is similar in structure to the ten tacles themselves, and is in like manner provided with tactile appendages, and with prehensile sagittae upon its external surface. These lips, by their contraction, shut and open the mouth at the pleasure of the Hydra, and, when the size of the animal is taken into the account, appear to be endowed with ex traordinary muscular force. The rest of the body is quite devoid both of sagittm and of tactile organs.
The body of the Hydra, according to Corda, is covered externally with a membrane that consists of two layers, of which the exterior (fig. 29, a) is composed of large cells, whilst in the inner layer are contained the germina, of which we shall immediately have to make further mention.
Between the skin and the alimentary canal Corda announces the existence of a muscular layer (fig. 29, b), composed of dense cells, which are coloured, and appear to be filled with minute granules.
The innerniost layer of all (fig. 29, e), from its position and texture, ought, accord ing to the same author, to be called the villous coat Onnica villo8a). This stratum lines the entire alimentary tract, from the margin of the labial processes as far as the arms, being divided at intervals by folds into numerous compartments. The villi of which this stratum consist are intimately connected with the muscular layer ; their shape is cylin drical, but they are of two kinds, some being perforated at their apex by a foramen, whilst the others are close. Each of these villi (fig. 28, 4) is a rounded, pellucid vesicle, the walls of which are thick, and probably con tractile, and iq those which are perforated the perforation would seem to convey nu tritive matter into their interior.
Behind the anal orifice there is a small, hollow, and contractile membranous pro longation (fig. 29, e), which constitutes the sucker, or foot.