COPEPODA 339 



ORDER 2. COPEPODA.* 



Body elongated, the thorax and abdomen being usually distinctly 

 segmented, and made up of 15 somites, 5 of which are united to form the 

 head and 5 form each the thorax and the abdomen. The head and thorax 

 together form the cephalothorax. In many forms fusion takes place 

 between the head and the first thoracic somite, or between the fourth 

 and the fifth thoracic somites, so that but 4 free thoracic segments are then 

 present. The abdominal somites are also often united in the female, the 

 first and second invariably. The last abdominal segment ends with 

 the furca, a pair of terminal projections bearing a definite number of long 

 caudal bristles. In the parasitic copepods the form and structure of the 

 body have often been profoundly changed, and all semblance to the typical 

 copepod form may have been lost. Ten pairs of appendages are present, 

 5 of which are cephalic and 5 thoracic. The first pair of antennae 

 is uniramose and is longer than the second and may be used for locomo- 

 tion; in the male either one or both of the first pair are often modified 

 to form clasping organs. The second pair is biramose (but occasionally 

 uniramose) and sometimes provided with prehensile hooks and spines. 

 A poisonous sting is present in front of the mouth in Argulus. The 

 thoracic appendages are biramose swimming legs (Fig. 542,6), which are 

 without gills. Except in the Pontellidae and the Argulidae a single 

 median eye is present. 



The genital openings are in the first abdominal segment: except in 

 the Argulidae the female carries her eggs in 1 or 2 gelatinous masses, 

 the so-called egg sacs, which project from the segment into the water. 

 The young animal is born as a nauplius. 



More than 2,200 species of copepods are known, of which nine-tenths 

 are marine. About half of these are non-parasitic and form one of the 

 most important elements in the plankton, because of their enormous 

 numbers: they are among the most important scavengers of the sea 

 and form besides the principal food of herrings and many other fishes. 

 The parasitic copepods are called fish lice and live on the external surface 

 and gills of fishes and occasionally on squids and other animals. The 

 order contains about 20 families grouped in 2 suborders. 



* See "Die freilebenden Copepoden," etc., by C. Glaus, 1863. "Deutschlands 

 freilebende Siisswasser Copepoden," by O. Schmeil, Bibliotheca Zoologica, 1892-1896. 

 "Synopsis of the Entomostraca of Minnesota," etc., by C. L. Herri ck, Sec. Rep. of 

 State Zool., 1895. "Copepoda of the Woods Hole Region," by W. M. Wheeler, Bull. 

 U. S. Fish. Com. for 1899, Vol. 19, p. 157. "Freshwater Copepoda of Mass.," by 

 A. S. Pearse, Am. Nat., Vol. 40, p. 241, 1906. "Notes on Marine Copepoda of R. I.," 

 by L. W. Williams, Am. Nat., Vol. 40, p. 639, 1906. "Siisswasserfauna Deutschl.," 

 Heft 11, 1909. "Notes on the Marine Copepoda and fladocera of Woods Hole," etc., 

 by R. W. Snarpe, Proc. U. S. Nat. Mus., Vol. 38, p. 405, 1911. 



