Copepoda

Copepoda, an order of Crustacea (q.v.) belonging to the sub-class Entomostraca (q.v.). The characters by which they may be distinguished from allied orders are as follows :- (1) They are small but elongated in form; (2) they have four or five pairs of two-branched swimming feet (from which they derive their name, it signifying "oarfooted"); (3) the abdomen is composed of five segments; (4) the free-swimming forms are segmented, but this is lost in the degraded parasitic forms. The Copepoda live both in fresh and salt water; the best known genus is "Cyclops" (q.v.), one of the commonest and most easily recognised of all the "water fleas." Cetochilus is a well-known marine form, and occurs in such abundance in the colder regions of the Atlantic as to give the sea a reddish colour; it also serves as one of the main sources of the food of the whales. Many members of the order are parasitic, such as Lernaea, Argulus, etc., and these are very degenerate; their limbs are rudimentary, they have a suctorial mouth, and have usually lost their eyes and respiratory organs. Their close affinity to the other members of the class is shown by their passing through the same" Nauplius" (q.v.) stage, and as some of the males do not become parasitic they remain as normal Copepoda. In addition to the respiration carried on by the general surface of the body (cutaneous), there is also an "anal respiration," water being alternately admitted into and expelled from the rectum.