4V)2 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXVIII. 



differentiated from the remainder of the genital segment (5, fig. 1). 

 As development progresses these lobes become assimilated more and 

 more with the bod}^ of the segment, until at the lastthe}^ are oftentimes 

 invisible except from the ventral surface, and then only after careful 

 examination. 



Owing to this extreme variation in size and shape the greatest care 

 must be exercised in comparing different specimens for purposes of 

 classification. The individuals compared must be alike in sex, in 

 maturity, and even in the period of pregnancy if the size or shape of 

 the genital segment is to have its full significance. Fortunatety, one 

 breeding season follows another so rapidly that the female is never 



left for any long interval without her 

 e g'£ strings. Hence, in collecting these 

 parasites, fully ripe females are very 

 largely predominant. On being pre- 

 served the egg cases become very brit- 

 tle and break off easily, but 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 must not 

 be given too much prominence. 



The length of the egg strings and 

 the size of the eggs vary greatly in 

 different species and in different indi- 

 viduals of the same species, and the 

 best that can be done is to give the 

 general average. The size of the eggs 

 is always a better guide than the 

 number. 



Like the genital segment, the abdo- 

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

 occurring more frequently in the male. There are two species of 

 Caligus also in which it is three-jointed, C. coryphsenae and C. angus- 

 tatus, and another in which it is four-jointed, C. aliuncus. (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, 

 fig. 1). These processes have been given different names by different 

 authors. Milne Edwards calls them "lames caudales;" Kroyer desig- 

 nates them as "halevedhaengene" in Danish, while in his Latin 

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



Pig. 1.— Young female of lepeophtheirus 

 edwardsi, showing the fifth pair of 

 swimming legs at the posterior cor- 

 ners of the genital segment, a. i., anal 

 laminae; 5, rudimentary fifth legs. 



