280 



SEXUAL DIMORPHISM 



on this side are obsolete, except the most anterior. The 

 sides of the body are provided with very large scales, 

 not arranged in pairs, which fall backwards so as to cover 

 the body (probably these are the scales of the legs of one 

 side). The male is extremely minute, and lives partially 

 immersed between the folds of the posterior part of the body 

 of the female. 



In Athelges, although the position of the ovigerous 

 scales on the dorsal surface of the body is the same, 

 the body is almost perfectly symmetrical, a fact which is 

 most likely explained by the other fact that the animal 

 lives in the branchial cavity of a hermit crab, Pagurus, 

 instead of a shrimp. The carapace of the Pagurus is 

 soft. 



The genus lone is parasitic upon Callianassa, and although 

 the male is not very remarkable, and the proportions of the 

 sexes are as usual, the adult female exhibits peculiar charac- 

 ters in the branched abdominal appendages, and the simple 

 membranous appendages arising from the six anterior pairs 

 of legs. 



Copepoda. The typical Copepoda are minute sym- 

 metrical Crustacea, in which the body consists of two obviously 

 distinct parts, a large anterior portion provided with several 

 pairs of legs, and an abdomen or tail without appendages. 

 The anterior portion consists of a cephalothorax and four free 

 segments. The former bears two pairs of antennae, a pair of 

 mandibles, two pairs of maxillae, and a pair of long append- 

 ages similar to those which follow, namely, the four pairs 

 belonging to the free segments. These appendages are 

 biramous, and function as swimming legs in those species 

 which lead an active existence. The narrow cylindrical tail 

 consists of five segments ending in a fork, and in the female 

 the two first of these are united, and carry the genital 

 openings. The Copepoda, like the fabled Cyclops whose 



