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. =Dorsal Organ.=—A structure very characteristic of adult Phyllopods is the “dorsal organ” (Figs. 2, 5, _D.O_), whose function is in many cases obscure. It is always a patch of modified cephalic ectoderm, supplied by a nerve from the anterior ventral lobe of the brain on each side; but its characters, and apparent function, differ in different forms. In the Branchipodidae the dorsal organ is a circular patch, far forward on the surface of the head (Figs. 2, 5, _D.O_). Its cells are arranged in groups, which remind one of the retinulae in a compound eye; each cell contains a solid concretion, and the concretions of a group may be so placed as to look like a badly-formed rhabdom. Claus,[16] who first called attention to this structure in the Branchipodidae, regarded it as a sense-organ. In Apodidae the dorsal organ is an oval patch of columnar ectoderm, immediately behind the eyes; it is slightly raised above the surrounding skin, and is covered by a very delicate cuticle (with an opening to the exterior?), and below it is a mass of connective tissue permeated by blood; Bernard has suggested that it is an excretory organ.