(177). The Hydrae:, or freshwater polyps, are common in the ponds and clear waters of our own country; they are generally found creeping upon confervae or submerged twigs, and may readily be procured in summer for the purpose of investigating the remarkable circumstances connected with their history.

(178). The body of one of these simple animals consists of a delicate gelatinous tube, contracted at one extremity, which is terminated by a minute sucker, and furnished at the opposite end with a variable number of delicate contractile filaments, placed around the opening that represents the mouth.

(179). When the Hydra is watching for prey, it remains expanded (fig. 39, 1, 2, 5), its tentacula widely spread and perfectly motionless, waiting patiently till some of the countless beings that populate the stagnant waters it frequents are brought by accident in contact with it: no sooner does an animal touch one of the filaments than its course is arrested as if by magic; it appears instantly fixed to the almost invisible thread, and in spite of its utmost efforts is unable to escape; the tentacle then slowly contracts, and others are brought in contact with the struggling prey, which, thus seized, is gradually dragged towards the orifice of the mouth, that opens to receive it, and slowly forced into the interior of the stomach.

Hydrozoa 52

Fig. 39.

(180). To the earlier observers of the habits of the Hydra nothing could be more mysterious than this faculty, possessed by the creature, of seizing and retaining active prey, in spite of all its efforts at resistance, but which is now satisfactorily explained as depending upon the presence of a prehensile apparatus, allied in its nature to the filiferous capsules of the Actiniae, described in the last chapter. These wonderful organs are not only thickly dispersed over the whole surface of the tentacles, but are likewise met with, though less numerously distributed, over the general surface of the body. They appear, under high powers of the microscope, to be composed of minute oval vesicles, from each of which can be protruded a long delicate filament, having its free extremity slightly swollen, and apparently of a soft viscid texture, the whole being not inaptly compared by Agassiz to a lasso. The neck of each vesicle is furnished with three recurved hooklets, which, when the skin of the animal is irritated, or when the arms are prepared to seize prey, remain erect and prominent.

The modus operandi of these structures is as simple as the result is efficacious: the "lasso-threads" with their viscid extremities, speedily involve the victim seized, in their tenacious folds, and closely bind it against the hooklets wherewith the surface of the tentacula is thickly studded: these, probably, in their turn constitute prehensile organs, and moreover form an apparatus of poison-fangs of a very deadly character; for it is observable that an animal once seized by the Hydra, even should it escape from its clutches, almost immediately perishes.

(181). Arrived in the stomach of the polyp, the animal that has been swallowed is still distinctly visible through the transparent body of the Hydra, which seems like a delicate film spread over it (fig. 39, 4); gradually the outline of the included victim becomes indistinct, and the film that covers it turbid; the process of digestion has begun: the soft parts are soon dissolved and reduced to a fluid mass, and the shell or hard integument is expelled through the same aperture by which it entered the stomach.

(182). No traces of vessels of any kind have as yet been detected in the granular parenchyma of which the creature is composed; coloured globules are seen floating in a transparent fluid, which, in the Hydra viridis, are green, although in other species they assume different tints. When the food has been composed of coloured substance, as, for example, red larvae, or black Planariae, the granules of the body acquire a similar hue, but the fluid wherein they float remains quite transparent; each granule seems like a little vesicle into which the coloured matter is conveyed, and the dispersion of these globules through the body gives to the whole polyp the hue of the prey it has devoured; sometimes the granules thus tinted are forced into the tentacula, from whence they are driven again by a sort of reflux into the body, producing a kind of circulation, or rather mixing up, of the granular matter, and distributing it to all parts. If, after having digested coloured prey, the polyp is made to fast for some time, the vesicles gradually lose their deepened hue and become comparatively transparent.

The granules, therefore, would seem to be specially connected with the absorption and distribution of nutriment.

(183). When mature and well supplied with food, minute gemmules or buds are developed from the common substance of the body; they spring from no particular part, but seem to be formed upon any portion of the general surface. These gemmae appear at first like delicate gelatinous tubercles upon the exterior of the parent polyp; but as they increase in size they gradually assume a similar form, become perforated at their unattached extremity, and develope around the oral aperture the tentacles characteristic of their species.

(184). This mode of propagation, termed "gemmation," differs from the development of the Hydra ah ovo, inasmuch as the germ-cell, which sets on foot the process, is derivative and included in the body of the adult, instead of being primary and included in a free ovum.

(185). Sometimes six or seven gemmae have been observed to sprout at once from the same Hydra; and although the whole process is concluded in twenty-four hours, not unfrequently a third generation may be observed springing from the newly-formed polyps even before their separation from their parent: eighteen have in this manner been seen united into one group; so that, provided each individual, when complete, exhibited equal fecundity, more than a million might be produced in the course of a month from a single polyp.