In the APODIDAE[15] the head is broad and depressed, the ventral side being nearly flat, the dorsal surface convex; the hinder margin of the head is indicated dorsally by a transverse cervical ridge, bounded by two grooves, behind which the carapace projects backwards as a great shield, covering at least half the body, but attached only to the back of the head. In _Lepidurus productus_ the head and carapace together form an oval expansion, deeply emarginate at the hinder, narrower end, the sides of the emargination being toothed. The carapace has a strong median keel. The kidneys project into the space between the folds of skin which form the carapace, and their coils can be seen on each side, the terminal part of each kidney-tube entering the head to open at the base of the second maxilla. In all Branchiopoda with a well-developed carapace the kidney is enclosed in it in this way, whence the older anatomists speak of it as the “shell-gland.” [Illustration: FIG. 2.—_Chirocephalus diaphanus_, female, × 5, Sussex. _D.O_, Dorsal organ; _H_, heart; _Ov_, ovary; _U_, uterus; _V_, external generative opening. ] Associated with the development of the carapace, in this and in the next family, is a remarkable condition of the lateral eyes, which are sessile on the dorsal surface of the head, and near the middle line, the median eye being slightly in front of them. During embryonic life a fold of skin grows over all three eyes, so that a chamber is formed over them, which communicates with the exterior by a small pore in front. In the LIMNADIIDAE the body is laterally compressed, and the carapace is so large that at least the post-cephalic part of the body, and generally the head also, can be enclosed within it. [Illustration: FIG. 3.—_Limnetis brachyura_, × 15. (After G. O. Sars.) ] In _Limnetis_ (Fig. 3) the dorsal surface of the head is bent downwards and is much compressed, the carapace being attached to it only for a short distance near the dorsal middle line. The sides of the carapace are bent downwards, and their margins can be pulled together by a transverse adductor muscle, so that the whole structure forms an ovoid or spheroidal case, from which the head projects in front, while the rest of the body is entirely contained within it. When the adductor muscle is relaxed the edges of the carapace gape slightly, like the valves of a Lamellibranch shell, and food-particles are drawn through the opening thus formed into the ventral groove by the movements of the thoracic feet, locomotion being chiefly effected by the rowing action of the second antennae, as in the Cladocera, to which all the Limnadiidae present strong resemblances in their method of locomotion, in the condition of the carapace, and in the form of the telson. In _Limnadia_ and _Estheria_ the carapace projects not only backwards from the point of attachment to the head, but also forwards, so that the head can be enclosed by it, together with the rest of the body.