NO. 140-1. PARASITIC COPEPODS'-CALIOIDj^^WILSON. 491 



fork between the bases of the second maxillipeds. The lamina of the 

 third swimming legs also renders effective service in this direction. 



}>. These genera show a decided preference for the Pleuronectidte, 

 and the (xadidffi as hosts, but such of them as practice free swimming 

 ma}' be found upon almost an}- host temporarily. Many of the species, 

 so far as obsei'ved, are confined to a single host. 



]0. These parasites feed upon the blood of their host in a similar 

 manner to the Argulida'. They are very susceptible to heat, and an 

 increase of temperature of only a few degrees is quickl}" fatal. On 

 the contrary, they can withstand very severe cold, even freezing, 

 without apparent injury. 



MORPHOLOGY. 



A. Ki'ternal. — The types upon which Miiller founded his genus CaJl- 

 gus in 1785 included several genera beside the true ( \d!guH. Indeed, the 

 only species amongst his types really belonging to the genus was Cal/gu>< 

 curtns. Hence his genus diagnosis was very broad and would have 

 included practically all our North American Caligina?. In the present 

 morphology the statements have been made equally inclusive and are 

 to be understood as embracing all North American Caliginse unless 

 otherwise limited. The body of a Caligus is made up of four parts or 

 sections, a cephalo-thorax, a free thorax, a genital segment, and an 

 abdomen. The cephalon bears seven pairs of appendages, namely, 

 antennules, antennae, mandibles, first and second maxilla?, and first and 

 second maxillipeds. The three anterior thoracic segments are fused 

 with the cephalon so that the cephalo-thorax bears three pairs of 

 swimming legs in addition to the appendages just enumerated. The free 

 thorax consists of a small segment carrying the fourth pair of swimming- 

 legs and the genital or reproductive segment. The latter has in both 

 sexes a pair of appendages which in the male are very evidentl}' rudi- 

 mentary swinnning legs of the fifth pair. In the female the}' are often 

 so reduced as to be recognized with difticulty, Init their presence is sufii- 

 cient to show that this segment must be regarded as a portion of the 

 thorax if we are to be consistent in our nomenclature of the Crustacea. 

 Hence, while retaining the designation ''genital segment,'' already in 

 general use, it will bo understood that this is really the fifth thoracic 

 segment, the second free one. It varies greatly in shape in diffei'ont 

 spc^cies, in different stages of development in the same species, and in 

 the two sexes. In the male and in immature females it is always 

 smaller and often approximates the abdomen closely in size. But as 

 th(> female approaches maturity it increases greatly and becomes 

 usually much larger than the abdomen. 



Furthermore, in undeveloped forms of both sexes the rudimentary 

 fifth legs are relatively nmch larger than in the mature individual, and 

 may commonly be seen as a pair of large lobes or processes clearly 



