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.
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.
In all these genera the carapace is flexible along the middle dorsal line; in Estheria especially the softening of the dorsal cuticle goes so far that a definite hinge-line is formed, and this, together with the deposition of the lateral cuticle in lines concentrically arranged round a projecting umbo, gives the carapace a strong superficial likeness to a Lamellibranch shell, for which it is said to be frequently mistaken by collectors.
The eyes of the Limnadiidae are enclosed in a chamber formed by a growth of skin over them, as in Apodidae, but the pore by which this chamber communicates with the exterior is even more minute than in Apus. The paired eyes are so close together that they may touch (Limnadia, Estheria) or fuse (Limnetis); they are farther back than in the Apodidae, while the ventral curvature of the head causes the median eye to lie below them. In all these points the eyes of the Limnadiidae are intermediate between those of Apus and those of the Cladocera.