The alimentary canal is extremely simple. The labrum is large, and forms a chamber above the mouth, into which food is driven by the limbs, as in the Phyllopoda, food being taken while the animal swims or lies on its back. The oesophagus runs vertically to join a small stomach, which bends sharply backwards and passes gradually into an intestine. In the last segment of the abdomen the intestine joins a short, thin-walled rectum, provided with radial muscles, by means of which it can be dilated. The dilatation of the rectum leads to an inhalation of water through the anus, which may possibly serve as a means of respiration. In the Daphniidae and Bosminidae there are two forwardly-directed digestive glands which open into the stomach, and in Eurycercus there is a large caecum at the junction of the rectum with the intestine. The intestine is usually straight, but in Lynceidae and in some Lyncodaphniidae it is coiled (e.g. Peracantha, Fig. [14]).
In Leptodora the alimentary canal is altogether remarkable; the oesophagus is a long and very narrow tube, which runs back through the whole length of the thorax and joins the mid-gut in the third abdominal segment. The mid-gut is not differentiated into stomach and intestine; it has no diverticula of any kind, and runs straight backwards to join the short rectum a little in front of the anus.
Fig. [14].—Peracantha truncata, female, × 100. Oxford.
The heart is always short, and never has more than a single pair of lateral openings; it is longest in the Sididae, which show some approximation to the Phyllopods in this, as in the slight degree of difference between their anterior and posterior thoracic limbs. The pericardium lies in the one or two anterior thoracic segments, dorsal to the gut. From the heart the blood runs forwards to the dorsal part of the head, and passes backwards by three main channels, one entering each side of the carapace, while the third runs down the body, beneath the alimentary canal to dilate into a large sinus round the rectum. This ventral blood-channel gives a branch to each limb, which forms a considerable dilatation in the epipodite, the blood from the limb returning to the pericardium by a lateral sinus. From the rectum a large sinus runs forwards to the pericardium along the dorsal wall of the body. The blood which enters each half of the carapace is collected in a median vessel and returned through this to the pericardium.
Those spaces between the viscera which are not filled with blood are occupied by a peculiar connective tissue, consisting of rounded or polyhedral cells, charged with drops of a fatty material which is often brightly coloured.
The reproductive organs are interesting because of the peculiar phenomena connected with the nutrition of the two kinds of eggs. The ovaries or testes are epithelial sacs, one on each side of the body, each continuous with a duct which opens to the exterior behind the last thoracic limb. In the female, the opening is dorsal (Fig. [10]), in the male it is ventral (Fig. [11]). The external opening is usually simple; but in the male there is sometimes a penis-like process, on which the vas deferens opens (Daphnella).
The eggs are of two kinds, the so-called “summer-eggs,” with relatively little yolk, which develop rapidly without fertilisation, and the so-called “winter-eggs,” containing much yolk, which require to be fertilised and then develop slowly.
At one end of the ovary, generally that nearest to the oviduct, there is a mass of protoplasm, containing nuclei which actively divide; this is the germarium (Fig. [15], A, B, C). As a result of proliferation in the germarium, nucleated masses are thrown off into the cavity of the ovary; each such mass contains four nuclei, and its protoplasm soon becomes divided into four portions, one round each nucleus, so that four cells are produced. In the simpler ovaries, such as that of Leptodora (Fig. [15], A), these sets of four cells are arranged in a linear series within the tube of ovarian epithelium; in other cases, as in Daphnia, the arrangement is more irregular. In the normal development of parthenogenetic eggs, one cell out of each set of four becomes an ovum, the other three feeding it with yolk and then dying. Weismann[[32]] has shown that the ovum is always formed from the third cell of each set, counting from the germarial end, so that in the ovary of Leptodora drawn in Fig. [15], A, the ova will be formed from the cells marked E1, E2, E3. At certain times, one or two sets of germinal cells fail to produce ova; the epithelial wall of the ovary thickens round these cells, so that they become incompletely separated from the rest in a so-called “nutrient chamber” (Fig. [15], B, N.C). Germ-cells enclosed in a nutrient chamber degenerate and are ultimately devoured by the ovarian epithelium. The significance of these nutrient chambers is unknown.
