KEY TO THE ENTOMOSTRACA OF COLORADO 



287 



of valves more pronounced than in preceding species and not rounded. 

 Post-abdomen rounded. Length, 0.35 mm. Alonella exigua (Lilljeborg) 



Tolland; g.soo ft. [Wisconsin, Michigan. All parts of Europe.] 



99 (88) Outline of body as seen from the side, nearly circular. Genus Chydorus 

 Only one species in Colorado. Length, 

 0.5 mm. Chydorus sphaericus (O.F.M.) 



Found in all parts of state where collections have been made. 

 [The most widespread cladoceran, being found all over the world.] 



100 (i, 139) Five pairs of biramous, non-foliaceous 

 feet. Body moderately elongated and distinctly 

 segmented. No paired eyes. Antennae usually 

 of good size. Sub-class COPEPODA 



fst orfttnna 



,^- td antenna 



- /iand/bte 

 /st max/ifa 



- Zd maxilla 

 "- MQxWiped 



Cephahiijorax 



z. S'^/mmin^ Thet 



5th feet 



- Seminal receptacle 

 Egg -sac 



Abdomen 



furca 



~ Loteroi seto 

 lerrninol setae 



Fig. 60.— Female Cyclops. Ventral view. (After R. Hertwig.) 



Fig. 59. — Chydorus sphaericus. 



Next to the Cladocera, the Copepoda 

 Dim the most important part of our fresh- 

 water plankton Crustacea. The general 

 form of the body and the number and 

 nature of the appendages are shown in 

 Fig. 60 and Table I. To determine the 

 species of Copepods it is necessary to 

 make dissections to separate out the neces- 

 sary parts. 



Males and females are about equally 

 common. They may be readily distin- 

 guished by the fact that the males have 

 one or both antennae enlarged and genicu- 

 late, to serve as a clasping organ. The 

 fifth feet (sixth trunk limbs) are of im- 

 portance in determining species. They 

 are always reduced and are usually alike 

 in male and female, though in Diaptomus 

 they differ in the two sexes. The eggs 

 during development are carried by the 

 female in one or two brood-pouches at- 

 tached to the abdomen. The young 

 emerge from the egg in the nauplius stage 

 (Fig. 61). Very many nauplei as well as 

 other immature forms are often found in 

 collections in so great numbers as to 

 cause confusion to the beginner. They 

 cannot be determined by these keys and 

 it is possible for the student merely to 

 remember that they are young forms' 

 probably of some one of the mature forms 

 found in the same collection. 



loi (134) Abdomen much more slender than the cephalothorax, from which 

 it is sharply separated. . 102 



