KEY TO CLASSES AND ORDERS OF ANIMALS 23 



3(2, i) Always of small size. With three pairs of jointed 

 appendages used as swimming organs; middle pair 

 biramous; body disk-shaped or somewhat elongated 

 and triangular in outline in young individuals; m 

 later stages segments are developed at the posterior 

 end which form a narrow outgrowth that may bear 



small appendages. , 



Class Crustacea (Nauplms larvae) 



Subclass ENTOMOSTRACA 

 1(6) Free swimming or parasitic on fish. * 



2(5) Body usually distinctly segmented in free-living 

 forms. ^ 



3(4) Appendages of trunk flattened, leailike, ten or more 

 (suborder Branchiopoda) or four to six pairs (suborder 

 Cladocera). Carapace well developed and bivalve, 

 or small or rarely absent. The chief organs of locomo- 

 tion are either the trunk appendages (Branchiopoda) 

 or the enlarged second antennae (Cladocera). Mostly 

 in fresh water. (Pratt, 329; W. and W., 661 and 676.) 



Order Phyllopoda 



4(3) Appendages of body not flattened; carapace absent; 

 thorax with four or five pairs of two-branched (bira- 

 mous) appendages; abdomen without appendages. 

 Fresh water and marine and parasitic (parasites 

 greatly modified). (Pratt, 3391 W. and W., 741 •) 



Order Copepoda 



5(2) Body indistmctly segmented; free-swimming and 

 creeping forms. Body^ inclosed in a carapace consist- 

 ing of articulated right and left valves. Abdomen 

 rudimentary. Five pairs of appendages from the 

 head and two from the trunk present. Fresh water or 

 marine. (Pratt 3S2; W. and W., 806.) 



Order Ostracoda 



