II STRUCTURE OF LIMNADIIDAE 21 
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 
shehtly, like the valves of 
a Lamellibranch shell, and 
food - particles are drawn 
through the opening thus 
formed into the ventral 
groove by the movements 
; Fie. 3.—Limnetis brachyura, x 15. 
of the thoracic feet, loco- Osten Ch Olcaraly | 
motion 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 Hstheria 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 con- 
centrically 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 
