I 



124 TWELFTH ANN^UAL REPORT. 



CHAPTER III. 



ORDER COPEPODA. 



This extensive order contains minute and predominatingly pre- 

 daceous animals which constitute no inconsiderable part of the 

 fauna of fresh and salt waters. They serve a beneficent purpose 

 both as scavengers and as providing food-supply for the fry of fishes 

 and other aquatic animals. 



Oopepods are never enclosed in a bivalved shell but ordinarily 

 exhibit a more or less elongated cylindrical form composed of two 

 obvious sub-divisions. There are a few species which, by the great 

 prolongation and expansion of some of the tergites or dorsal shields, 

 seem to simulate shelled Crustacea. The anterior part of the body, 

 or cephalothorax, is composed of ten somites which are frequently 

 considerably united or fused. Five of these segments constitute 

 the head and bear respectively the following appendages: first, a 

 pair of several- to many-jointed antennse which are never prim- 

 arily sensory in function, although they usually are provided with 

 sense hairs or other like organs; second, a pair of two-branched an- 

 tennules, 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 function; fifth, a pair of maxillipeds 

 which not infrequently subdivide in later life to form what appear 

 to be two distinct pairs. 



The five thoracic segments have each a pair of swimming feet 

 consisting typically of a two-jointed base and two like, three-jointed 

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

 development of the inner 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 setse constituting, in many forms, a tail-fin or spring. 



