ORDER COPEPODA. 



This extensive order contains minute and predominatingly preda- 

 ceoiis animals which constitute no inconsiderable part of the fauna of 

 fresh and salt waters. They serve a beneficent purpose both as scav- 

 engers and as providing lood supply for the fry of fishes and other 

 aquatic animals. 



Copepoda are never inclosed in a bivalved shell, but ordinarily 

 exhibit a more or less elongated cylindrical form, composed of two 

 obvious subdivisions. There are a few species which, by the great pro- 

 longation and expansion of some of the tergites or dorsal shields, seem 

 to simulate shelled Crustacea. The anterior part of the body, or ceph- 

 alothorax, is composed of ten somites which are frequently consider- 

 ably united or fused. Five of these segments constitute the head and 

 bear respectively the following appendages: first, a pair of several- to 

 many-jointed antennae, which are never primarily sensory in function, 

 although they usually are provided with sense hairs or other like 

 organs; second, a pair of two-branched antennules, which sometimes 

 become almost simple or prehensile; third, a pair of mandibles in the 

 form of masticatory or piercing organs, these being usually provided 

 with a palpus; fourth, a pair of maxillae of various form and func- 

 tion; fifth, a pair of maxillipeds which not infrequently subdivide in 

 later life to form what apj)ear to be two distinct pairs. 



The five thoracic segments have each a pair of swimming feet con- 

 sisting typically of a two-jointed base and two similar, three-jointed 

 rami. The symmetry is frequently broken by the retardation of the 

 development of the inner or outer ramus, while the fifth pair of feet 

 may become rudimentary and in various ways subserve the organs of 

 sex. The five abdominal segments are nearly devoid of appendages 

 and are continued posteriorly by two caudal stylets which bear strong 

 setee, constituting, in many forms, a tail-fin or spring. 



All Copepoda^ even such as are, in later life, parasitic, begin their 

 existence as free-swinning nauplii. 



Though the vast majority of genera and species are marine, it 

 would seem that fresh -water Copepoda make up in the number of indi- 

 viduals what they lack in variety. 



