In Hydra the nettle-cells are of two distinct types, in one of which the thread is barbed at the base, whereas in the other it is simple. Both types have often two or more varieties and intermediate forms occur, but generally speaking the capsules with simple threads are much smaller than those with barbed ones. The arrangement of the nettle-cells is not the same in all species of Hydra, but as a rule they are much more numerous in the tentacles than elsewhere on the body, each large cell being surrounded by several small ones. The latter are always much more numerous than the former.
Capture and Ingestion of Prey: Digestion.
The usual food of Hydra consists of small insect larvæ, worms, and crustacea, but the eggs of fish are also devoured. The method in which prey is captured and ingested has been much disputed, but the following facts appear to be well established.
If a small animal comes in contact with the tentacles of the polyp, it instantly becomes paralysed. If it adheres to the tentacle, it perishes; but if, as is often the case, it does not do so, it soon recovers the power of movement. Animals which do not adhere are generally those (such as ostracod crustacea) which have a hard integument without weak spots. Nematocysts of both kinds shoot out their threads against prey with considerable violence, the discharge being effected, apparently in response to a chemical stimulus, by the sudden uncoiling of the thread and its eversion from the capsule. Apparently the two kinds of threads have different functions to perform, for whereas there is no doubt that the barbed threads penetrate the more tender parts of the body against which they are hurled, there is evidence that the simple threads do not do so but wrap themselves round the more slender parts. Nussbaum (Arch. mikr. Anat. xxix, pl. xx, fig. 108) figures the tail of a Cyclops attacked by Hydra vulgaris and shows several simple threads wrapped round the hairs and a single barbed thread that has penetrated the integument. Sometimes the cyst adheres to the thread and remains attached to its cnidoblast and to the polyp, but sometimes the thread breaks loose. Owing to the large mass of threads that sometimes congregate at the weaker spots in the external covering of an animal attacked (e. g., at the little sensory pits in the integument of the dorsal surface of certain water-mites) it is often difficult to trace out the whole length of any one thread, and as a thread still attached to its capsule is frequently buried in the body of the prey, right up to the barbs, while another thread that has broken loose from its capsule appears immediately behind the fixed one, it seems as though the barbs, which naturally point towards the capsule, had become reversed. This appearance, however, is deceptive. The barbs are probably connected with the discharge of the thread and do not function at all in the same way as those on a spear- or arrow-head, never penetrating the object against which the projectile is hurled. Indeed, their position as regards the thread resembles that of the feathers on the shaft of an arrow rather than that of the barb of the head.
Adhesion between the tentacles and the prey is effected partly by the gummy secretion of the glands of the ectoderm, which is perhaps poisonous as well as adhesive, and partly by the threads. Once the prey is fast and has ceased to struggle, it is brought to the mouth, which opens wide to receive it, by the contraction and the contortions of the tentacles, the column, and the peristome. At the same time a mass of transparent mucus from the gastral cavity envelops it and assists in dragging it in. There is some dispute as to the part played by the tentacles in conveying food into the mouth. My own observations lead me to think that, at any rate so far as H. vulgaris is concerned, they do not push it in, but sometimes in their contortions they even enter the cavity accidentally.
When the food has once been engulfed some digestive fluid is apparently poured out upon it. In H. vulgaris it is retained in the upper part of the cavity and the soluble parts are here dissolved out, the insoluble parts such as the chitin of insect larvæ or crustacea being ejected from the mouth. Digestion is, however, to a considerable extent intracellular, for the cells of the endoderm have the power of thrusting out from their surface lobular masses of their cell-substance in which minute nutritive particles are enveloped and dissolved. The movements of the cilia which can also be thrust out from and retracted into these cells, keep the food in the gastral cavity in motion and probably turn it round so as to expose all parts in turn to digestive action. Complete digestion, at any rate in the Calcutta form, takes several days to accomplish, and after the process is finished a flocculent mass of colourless excreta is emitted from the mouth.
Colour.
In Hydra viridis, a species that has not yet been found in India, the green colour is due to the presence in the cells of green corpuscles which closely resemble those of the cells of certain freshwater sponges. They represent a stage in the life-cycle of Chlorella vulgaris, Beyerinck[[AM]], an alga which has been cultivated independently.
In other species of the genus colour is largely dependent on food, although minute corpuscles of a dark green shade are sometimes found in the cells of H. oligactis. In the Calcutta phase of H. vulgaris colour is due entirely to amorphous particles situated mainly in the cells of the endoderm. If the polyp is starved or exposed to a high temperature, these particles disappear and it becomes practically colourless. They probably form, therefore, some kind of food-reserve, and it is noteworthy that a polyp kept in the unnatural conditions that prevail in a small aquarium invariably becomes pale, and that its excreta are not white and flocculent but contain dark granules apparently identical with those found in the cells of coloured individuals (p. 154).
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.