HYDROZOA.
The Hydrm are to be met with abun dantly in summer time in almost every pond or ditch, and may easily be collected along with the duck-weed or other aquatic plants among which they reside. On filling a glass jar with the water in which they reside, and allowing it to stand for a few hours undis turbed, the little polypes will be fdund, some times in great numbers, adhering to the sides of the vessel, in which position nothing is easier than to watch their proceedings, and with the assistance of a simple magnifier to verify the descriptions which Trembley and others have given of the extraordinary phe nomena they exhibit.
The Hydra viridis, or short-armed polype, which is the species most commonly met with in this country, resembles, when expanded, a little bit of green sewing-silk, about the sixth part of an inch in length, attached by one ex tremity to the interior of the jar, or to any other fixed body, and having the opposite end slightly untwisted.
When moderately magnified, the body of the animal is found to be a little bag open at one extremity, the opening, which is in fact its mouth, being surrounded with seven deli cate filamentary tentacula ; while the other end is provided with a little flattened disc or sucker, by which it fixes itself to any foreign body (fig. 25). Its substance scents to be entirely composed of a gelatinous mate rial, in which are contained numerous green ish granular particles suspended in a glairy its body : nevertheless it is highly contractile, shrinking, when disturbed, into an almost in visible jelly-like speck, and again slowly ex panding itself NN hen left quiet. Incapable as such a creature would appear to be of any active exertion, this little gelatinous bag is soon found to be gifted with a capability of locomotion, which is exercised in the fol lowing manner : whilst attached to the side of the glass by the sucker at its closed extre mity, which forms a minute foot, something like that of a Gasteropod Mollusk, the little polype gradually- inflects its body until some of the tentaeula around its mouth are brought in contact with the supporting surface, of which they take a firm hold ; in this position it detaches its posterior sucker, and, advanc ing it towards its head, again fixes it, and thus progresses, after the manner of a leech, by a repetition of the same manceuvres.
may, however, be easily imagined that, owing to the minute size of the Hydra, and the ex treme slowness of its contraction, this mode of progression is by no means remarkable for its speed, and in fact a march of an inch will occupy it many hours in its performance ; accordingly the polype has been endowed with another mode of transit, of which it can avail itself at pleasure. Although its body is spe cifically heavier than water, so that when de tached from its hold it sinks helplessly to the bottom, it is able, when occasion requires, to row itself about in a very ingenious manner. In order to accomplish this feat, it first creeps to the top of the water, and protruding its sucker to a little distance above the surface, hollows it out into a saucer-like cavity, the buoyancy of which is sufficient .to keep it afloat ; and then, supported by this curiously contrived boat, the little Hydra rows itself about by means of its tentacula in whatever direction it chooses. No traces of nervous matter are perceptible in the composition of the Hydra, which, in its whole structure, is completely acrite ; nevertheless it is evidently ' able to appreciate the presence or absence of light ; for if a number of these little animals are confined in a glass vessel, one side of which is exposed to light vvhile the other is kept in the shade, they are always found to congregate on the illuminated side, and by turning the glass round it will be found that by changing their position, they will endea vour to regain a situation exposed to the solar influences : seeing, therefore, that they are absolutely destitute of eyes, it would seem that they perceive light by the sense of touch alone.