492 



PR<)<'J<:EDlNaS OF THE NATIONAL MUSEUM. 



vol.. XXVIII. 



ditferentiated from the rcniuinder of the genital .segment (5, fig. 1). 

 As development progresses these lol)es ])ecome assimilated more and 

 more with the Ixxly of the segment, until at the last they are oftentimes 

 invisible except from the \'entral surface, and then only after carcd'ul 

 examination. 



Owing (() this extreme variation in size and shape the grc^atest care 

 uuist be exeivised in comparing ditt'ercnt specimens for purposes of 

 classification. The individuals compared must l)e alike in sex, in 

 matui'ity, and even in the period of pregnancy if the size or shape of 

 the genital segment is to ha\'e its full signiticance. Foi'tunately, one 

 breeding season follows another so rapidl}^ that the female is never 



left for an}" long interval without her 

 egif strings. Hence, in collecting these 

 parasites, fully ripe females are very 

 largely predominant. On being pre- 

 served the Qgg cases become very brit- 

 tle and break oti' easily, tmt examination 

 will quickly reveal the fact that they 

 have been present, which of course is 

 all that is required. In the Key which 

 is given later (see p. 555) the shape of 

 the genital segment is made one of the 

 final means of determination after the 

 other more important ones have been 

 exhausted, and even then it riuist not 

 be given too much prominc^nce. 



The length of the i}^^v^ strings and 

 the size of the eggs vary greatly in 

 different species and in different indi- 

 viduals of the siune s})ecies, and the 

 best that can be done is to o-ivo the 



fk;. 1.— Young female op i.ni'EoniTHErRrs o'eiieral averat'"e 



EDWAKDSI, SlIOWINO THE EIFTH I'AIIl OF "^ 

 SWIMMI.N(; l,E(;s AT THE POSTERIOR COl:- 

 NERSOF THE(;ENITAI, segment, a. L. ANAL 

 LAMIN.E; b, RUDIMENTARY FIFTH LEGS. 



The size of the eggs 



is always a l)etter guide than the 



number. 



Like the genital segment, the al)do- 

 men is usually simple, but sometimes two-jointed, this condition 

 occurring more fi'equently in the male. There are two species of 

 Caligus also in which it is three-jointed, 0. coj'i/j)/i^)i;e and C. av</ns- 

 tatus\ and another in which it is four-jointed, C. (illuucus. (See 

 Plates VII and IX.) 



The abdomen is terminated by two processes, one on either side of 

 the anus, and each furnished with three or four plumose seta' (a. 1, 

 tig. 1). These processes have been given diflerent names hy different 

 authoi's. Milne Kdwards calls them ^' lames caudales;" Krciyer desig- 

 nates them as '''halevedhaengene''' in Danish, while in his lyatin 

 diagnoses they are simply ''"appendices;" Heller speaks of them as 



