/. CRUSTACEA: COPEPODA. 417 



3. ESTHERIID.E. Body laterally compressed and enclosed in a bivalve 

 shell, compound eyes fused; male very rare. Estheria* Limnadia* in 

 fresh water. 



Order II. Cladocera. 



Like the estheriids the small Cladocera have the body enclosed in a 

 bivalve carapace, which, in some instances, is small and reaches back only 

 over the first trunk segments, in others is large, enclosing the body, with a 

 notch for the protrusion of the head, while behind it terminates in a sharp 

 spine. The head bears a pair of large swimming antennae and a much 

 smaller first pair bearing olfactory bristles and, in the male, hooks for 



FIG. 419. Branchipus vernalis,* fairy shrimp. (After Packard.) 



clasping the female. The body consists of few segments, the heart is a 

 simple sac, and the fused faceted eyes, with paired optic nerves, are capa- 

 ble of motion in a special optic capsule. 



The young eggs in the sexual organs always occur in groups of four 

 (fig. 420). Of these but one grows into an egg, the others breaking down 

 and serving this as nourishment. Larger eggs with more yolk occur when 

 several of these groups (2-12 fuse to form a single egg. The summer 

 eggs arise from a single group, the winter eggs from several groups of 

 primordial ova. 



In all Cladocera the space between the back of the animal and the 

 shell serves as a brood pouch. In some cases this pouch contains an 

 albuminous fluid secreted by the mother, which nourishes the embryo. 

 The larger winter eggs one or two in number frequently remain for 

 awhile in the brood chamber and are there enveloped in a peculiar shell, 

 the.ephippium, consisting of two chitinous plates, like watch crystals, their 

 edges closely appressed. 



DAPHNID^:. Shell weft developed; Daplinia* (fig. 420), Bosmina* 

 POLYPHEMID^E. Shell small, only functioning as a brood case; head with 

 an enormous eye and large swimming antenna; no phyllopodous feet; 

 marine and lacustrine. Leptodora hyalina * appears at night, sometimes 

 in great numbers, in some of our lakes. Evadne,* marine. 



Sub Class III. Copepoda. 



A general description of the copepods can only apply to the 

 non-parasitic forms, since many of the parasites have become so 



