Berninger[AN] has just published observations on the effect of long-continued starvation on _Hydra_ carried out in Germany. He finds that the tentacles, mouth, and central jelly disappear, and that a closed "bladder" consisting of two cellular layers remains; but, to judge from his figures, the colour does not disappear in these circumstances. [Footnote AN: Zool. Anz. xxxvi, pp. 271-279, figs., Oct. 1910.] BEHAVIOUR. _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 manoeuvre 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.