NO. 1504. AMERICAN PARASITIC COPEPODS— WILSON. 677 



as freely movable as any of the other thorax joints. The longitudinal 

 and transverse g-rooves are also f ull}- formed, so that the dorsal sur- 

 face presents the same areas as in the adult. 



The third and fourth thorax segments have changed but little; the 

 fourtli projects farther proportionally over the bases of the fourth 

 legs. There is a similar semiseparation of a fifth segment at the 

 anterior end of the genital segment, and the rudiments of a fifth pair 

 of legs can still be seen at its posterior corners. Neither the segment 

 nor the legs are as prominent as in the preceding stage, and in all 

 probability they soon disappear. 



But a radical change has taken place in the genital segment itself. 

 This has widened into a broad acorn shape, as wide anteriorly as pos- 

 teriorly, with the posterior corners projecting slightly backward and 

 showing the sixth legs plainly at their tips. The abdomen is narrow 

 with straight sides; the anal lamina3 are ver}' narrow and nearl}^ as 

 long as the abdomen itself, each armed with 1 long plumose setse. 



The appendages have now assumed their final form; the second 

 anteimtv are developed into powerful prehensile organs with long and 

 stout terminal claws. The first and second maxillipeds are about the 

 same size, and neither of them large enough to be of an}' real service 

 for prehension. The rami of the swimming legs have all become 

 clearly three-jointed, except the endopods of the fourth legs, which in 

 this species remain two-jointed in the adult, and the}" function as 

 powerful locomotor organs. The transition from this stage to the 

 adult is very slight and consists chiefly in the changes produced in the 

 genital segment liy the maturation of the eggs, the consequent enlarge- 

 ment of the oviducts, and the elimination of all traces of a separation 

 into fifth and sixth segments. 



The rudimentary fifth legs entirely disappear in the female, and 

 there is absolutely nothing left to indicate that the genital segment 

 contains more than a single thorax joint. 



Kroyer** notes that the form of the genital segment in all his speci- 

 mens of females diflers from that of the male, and keeps a sexual 

 peculiarity through all its changes, going over gradually from an 

 elongate-angular form into a flask shape. 



And he adds: " How far females younger than those I have examined 

 may present on this point an approximation to the males, I may leave 

 to the decision of future investigators." 



In the present instance the larval females, which are younger than 

 any he obtained, do not show "an approximation to the male," but 

 even the youngest of them has the distinctive angular form of its 

 own sex. 



« Bidrag til Kundskab om Snyltekrebsene, 1863, p. 152. 



