ORDER COPEPODA. 
This extensive order contains minute and predominatingly preda- 
ceous animals which constitute no inconsiderable part of the fauna of 
fresh and salt waters. They serve a beneficent purpose both as seav- 
engers and as providing food-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 antenne, 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 maxille of various form and fune- 
tion; 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 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 
sete, 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. 
