STRUCTURE OF LIMNADIIDAE 



21 



Fig. 3. — Livinetis brachyurUy x 15. 

 (After G. 0. Sars.) 



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, loco- 

 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 Ustheria 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 Ustheria 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 Apiis. The paired eyes are so close together that they 

 may touch (Limnadia, Ustheria) 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 



