276 



UNIVERSITY OF COLORADO STUDIES 



29 (28) Shell not greatly elongated. 30 



30 (31) Hinge-line straight, ending in a definite angle behind. Umbones 

 prominent and shell full. Length, 10-12 mm. Estheria morsei Va-ckard 



La Junta. [South Dakota, Nebraska, Colorado.] 



Fig. 16. — Estheria compleximanus. 



■Estheria mexicana. 



31 



32 



(30) Hinge-line slightly curved and not ending in an angle behind. Um- 

 bones less prominent than in above species, and shell flatter. Length, 

 10. 2 mm Estheria mexicana Claus 



La Junta, Greeley. [Lake Winnipeg to Mexico and from Ohio to the Rocky Mountains.l 



(25) Head not inclosed within valves, but with a separate shield, ending 

 in a prominent beak below. Valves without con- 

 centric growth markings. Family Limnetidae 

 Genus Limnetis 

 One species in Colorado. Length, 3 . o rmn. 



Limtieiis gouldii Baird 



Tolland region; 

 Canada.] 



8,500-9,500 ft. [Northern United States and 



Fig. 19. — Limnetis gouldii. 



(2) Post-cephalic appendages, 4-6. Body and 

 feet usually covered with a bivalve carapace. 

 Small animals usually less than 2 . o mm. in length. The "water fleas." 



Order Cladocera 34 



The key to the Cladocera is based upon the characters of the mature female. 



The Cladocera form a large part of the zooplankton of both salt and fresh water. They will be found 

 to constitute by far the greater part of plankton collections from most fresh-water lakes and pools. The 

 general structure of a typical cladoceran is illustrated by the drawing of Daphnia pulex (Fig. 26) and the 

 nature and number of the appendages are given in Table I. The trunk and its appendages, but not the head, 

 are inclosed within a bivalve carapace, of characteristic shape in each genus. The large eye with numerous 

 lenses, so conspicuous in the head, is composed of two lateral eyes fused into one mass. Below it, in nearly 

 all forms, is the ocellus, representing the nauplius eye. The constant motion of the trunk appendages keeps 

 a continuous current of water circulating between the valves, bringing in food and oxygen and removing 

 wastes. The food, chiefly of algae, is fed forward by the gnathobases of the legs to the mouth, where, within 

 the labrum, it is worked upon by the mandibles. The second antennae are powerful organs of locomotion. 



Back of the appendages, the body is continued as the freely movable post-abdomen with two terminal 

 daws used to remove foreign substances from among the legs. Near its base, on the dorsal side, it bears a pair 

 of long sensory hairs, the abdominal setae, and farther back, on either side, a row of teeth, the anal spines. 

 On each claw, near its base, are usually one or two spines, the basal spines; or a series of them, the pecten. 

 Between the dorsal surface of the trunk and the carapace is a space which, in the females, serves as a brood- 

 chamber. 



