Hydra viridis is a more sluggish animal than the other species of its genus and does not possess the same power of elongating its column and tentacles. It is, nevertheless, obliged to feed more frequently. Wagner (Quart. J. Micr. Sci. xlviii, p. 586, 1905) found it impossible to use this species in his physiological experiments because it died of starvation more rapidly than other forms. This fact is interesting in view of the theory that the green corpuscles in the cells of H. viridis elaborate nutritive substances for its benefit. H. vulgaris, at any rate in Calcutta, does not ordinarily capture prey more often than about once in three days.
All Hydræ (except possibly the problematical H. rubra of Roux, p. 160) spend the greater part of their time attached by the basal disk to some solid object, but, especially in early life, H. vulgaris is often found floating free in the water, and all the species possess powers of progression. They do not, however, all move in the same way. H. viridis progresses by "looping" like a geometrid caterpillar. During each forward movement the column is arched downwards so that the peristome is in contact with the surface along which the animal is moving. The basal disk is then detached and the column is twisted round until the basal disk again comes in contact with the surface at a point some distance in advance of its previous point of attachment. The manœuvre is then repeated. H. vulgaris, when about to move, bends down its column so that it lies almost prone, stretches out its tentacles, which adhere near the tips to the surface (p. 153), detaches its basal disk, and then contracts the tentacles. The column is dragged forward, still lying almost prone, the basal disk is bent downwards and again attached, and the whole movement is repeated. Probably H. oligactis moves in the same way.
When H. viridis is at rest the tentacles and column, according to Wagner, exhibit rhythmical contractions in which those of the buds act in sympathy with those of the parent. In H. vulgaris no such movements have been observed. This species, however, when it is waiting for prey (p. 154) changes the direction of its tentacles about once in half an hour.
All species of Hydra react to chemical and physical stimuli by contraction and by movements of the column and tentacles, but if the stimuli are constantly repeated, they lose the power to some extent. All species are attracted by light and move towards the point whence it reaches them. H. vulgaris, however, at any rate in India, is more strongly repelled by heat. Consequently, if it is placed in a glass vessel of water, on one side of which the sun is shining directly, it moves away from the source of the light[[AO]]. But if the vessel be protected from the direct rays of the sun and only a subdued light falls on one side of it, the polyp moves towards that side. No species of the genus is able to move in a straight line. Wilson (Amer. Natural. xxv, p. 426, 1891) and Wagner (op. cit. supra) have published charts showing the elaborately erratic course pursued by a polyp in moving from one point to another and the effect of light as regards its movements.
If an individual of H. vulgaris that contains half digested food in its gastral cavity is violently removed from its natural surroundings and placed in a glass of water, the column and tentacles contract strongly for a few minutes. The body then becomes greatly elongated and the tentacles moderately so; the tentacles writhe in all directions (their tips being sometimes thrust into the mouth), and the food is ejected.
Reproduction.
Reproduction takes place in Hydra (i) by means of buds, (ii) by means of eggs, and (iii) occasionally by fission.
(a) Sexual Reproduction.
The sexual organs consist of ovaries (female) and spermaries (male). Sometimes the two kinds of organs are borne by the same individual either simultaneously or in succession, but some individuals or races appear to be exclusively of one sex. There is much evidence that in unfavourable conditions the larger proportion of individuals develop only male organs.