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The latter are sometimes grouped together under the common designation of Branchiopoda, or the gill-footed division. To that division we now proceed.

To the third order, the Cladocera, belong the various species of Daphnia and its near relations. Let us first examine a full-grown Daphnia, preferably one of the larger species, say D. pulex or D. retula. We see a body composed of two parts, the head terminating below in a sort of beak, and a thorax and abdomen, the two last enclosed in a nearly transparent carapace. In D. pulex the carapace tapers off behind in a long dagger-shaped point. In D. macronata it bears two long spines at the posterior corners. In D. retula it is lopped off bluntly and slantingly forwards. In D. reticulata it is marked with a network of lines over its surface: and in Acroperus harpæ these lines resemble the strings of a harp. Below the beak are a pair of exceedingly small antennæ, so small that they may be easily overlooked. Springing, as it were, from the neck, we see a pair of vary large branched antennæ. These are the principal organs of locomotion. The eye consists of twenty crystalline lenses, or fewer, and is turned about on a cushion of appropriate tissue by two sets of muscles. This coarsely compound organ may be regarded as foreshadowing the highly perfect compound eye of insects. Just behind the eye may be seen the brain. At the junction of the head and body near the base of the beak, is situated the mouth, which opens into a short gullet, and that again into a roomy stomach with its two cæca; and then follows a straight intestine, which finally curves downwards towards the tail. Only the first segment of the body is adherent to the carapace, the rest moving quite freely between the valves. Ample room is thus afforded on the back of the animal for the accommodation of its eggs after they are laid; and there, accordingly, the eggs are carried about in a bundle till they are hatched. The tail, which terminates in two hooks, is used as a rudder and propeller. Its motions are very vigorous. If you look through the shell at that part of the body which succeeds the neck, you will notice an oval-shaped organ, which keeps regularly dilating and contracting. This is the heart. From its anterior extremity springs an artery, and on opposite sides of the heart are two slits which receive the blood from the surrounding cardiac chamber, and close up at each contraction while the blood is urged forwards. There are five pairs of legs, employed for other purposes than swimming. When the animal is at rest, they create currents in the water, and so bring food to the mouth. The first and second pairs are used as organs of prehension. The third and fourth pairs mainly subserve respiration, for which they are well adapted by their bronchial plates fringed with numerous filaments. I have already remarked upon the extraordinary fecundity of many of the Entomostraca. The Daphnia are no exceptions to the rule, and present similar phenomena of parthenogenesis. The development of the embryo may easily be watched through the thin carapace, and is, of course, extremely interesting. The first organ to show itself is the eye; on the fifth day from laying, the young Daphnium cone out, and then go through their series of moultings. One great advantage of the moulting process to the Entomostraca is, that they are thus able to rid themselves of troublesome infusorial parasites, which often lodge upon the carapace in great numbers, and seriously retard the movements and depress the vitality of their host. Bunches of Epistylis are very common upon Cyclops and Daphnia. Towards the approach of winter, the Daphnia have the remarkable habit of enveloping eggs in a special casing between the outer and inner layer of the carapace. This casing, which is developed on the back of the mother, has been called the ephippium, from its fancied resemblance to a saddle. It will easily be recognised by the microscopist. When the skin is cast, this structure is