METAZOA CRUSTACEA. 327 



rest in the water." At other times they are used as oars 

 and each powerful stroke propels the animal a consider- 

 able distance through the water. 



The second or posterior pair of antennae are divided 

 into two branches and supplied with long hairs. These 

 are also adapted for swimming. The mouth parts con- 

 sist of a pair of strong mandibles, one pair of maxillae 

 which carry long branchial filaments, and two pairs of 

 well developed maxillipeds. The five pairs of rowing 

 feet aid the antennae in locomotion, and thus it is seen 

 that the pelagic Copepods are preeminently swimmers. 



Besides the marine forms of Copepoda there are many 

 related fresh-water species, the most familiar of which is 

 Cyclops (PL 803, fig. i, C. brasiliensis, $ ; fig. 2, C. 

 vitiensis, 9 ). In the fresh-water forms the first pair of 

 antennae are much shorter than in Calanus, and conse- 

 quently the animals are not such good swimmers. The 

 setae of the second pair open and close like the fingers of 

 a hand, so that by means of these organs Cyclops can 

 attach itself to objects. In the middle of the forward 

 part of the cephalothorax there is an eye that appears to 

 be single but which in reality is double. 



The fifth pair of legs exist as vestiges and usually have 

 but one joint. 



The female (fig. 2) carries the two sacs of eggs about 

 with her for a time, and the young are hatched as nauplii. 



Nearly every group of Crustacea has its parasites illus- 

 trating specialization by the reduction of some organs 

 and the modification of others. The Copepoda are no 

 exception to this rule ; indeed, they offer some modified 

 forms which are far removed from their primitive ances- 

 tors. Such are Chondracanthus (No. 804 ; PI. 805) 

 which lives in the gill chamber of a fish ; Penella (No. 

 806) and Lernea (No. 807). 



The body of Chondracanthus (No. 804; PI. 805, 

 C. gibbosus, side view) has lost its distinct segmentation, 

 while the abdomen exists as a vestige only. The eyes 



