642 PROCEEDINCS OF THE NATIOXAL MUSEUM. voi. xxv. 



a modification of the various organs especially' used in parasitism — 

 e. g., organs for clinging to their host, for piercing after l)lood, etc., 

 as will the better adapt them to their specific use. 



Tlien we find forms like Cal'ujus which roam about freely over their 

 host's bod}' but do not apparently leave it voluntarily, though they can 

 swim well enough when compelled to do so. 



Here also we should not expect an}-^ marked degeneration, but rather 

 a more complete adaptation of the various organs. 



The first evidence of degeneration in this genus lies apparently in 

 the inclination toward free swimming and not so much in the abilit}^ 

 to perform it. In these two large genera, Argulus and Caligiis^ the 

 males and females differ but slightly, and in some species of A^'guhis 

 they may even be approximately of the same size. The fact that the 

 Argulid?e do not carry their eggs about with them tends still farther 

 to eliminate the sexual differences, while in the Caligida^ the presence 

 or absence of the long egg pouches with the attendant modifications of 

 structure constitute the chief sexual distinctions. 



Not so, however, in forms like Panda/'us, for here both sexes usually 

 fasten themselves in one place and remain there for a long time. They 

 are also so dissimilar in habits and structure that the males have been 

 hitherto placed in an entirely separate genus (JVogagns), and the two 

 sexes have been proved to belong to the same species only by being 

 repeatedl}' found in actual coition. As is usual in such cases the female 

 is the more degenerate and can only crawl about slowl}'; she is so 

 heavy and clumsy that she can not swim at all. 



Here then is evidence of structural degeneration, not very marked 

 as yet, since fully developed swimming organs are retained though they 

 can not be used in the adult state. 



The male Pandariis^ on the contrary, not only retains the locomotor 

 structures but can use them, being able to swim about f reel}' whenever 

 occasion demands. 



Again there are forms like PMlicthyx in which both the male and 

 female have become practically incapable of locomotion, but are still 

 found free in the mucous canals and sinuses of fishes. In them the 

 locomotor organs are markedly degenerate having dwindled to mere 

 stumps without joints or setfe. 



And, finall}', we find forms like Cho mini cant hus, Anchm^ella, and 

 Lernea, in which the female is absolutely incapable of motion, being- 

 fixed in one position for life, while the male has dwindled to a mere 

 pigm}' adherent to some part of the female's bod}-. The male can still 

 move about somewhat but the female has lost all trace of every append- 

 age except those which serve to fasten her to her host. 



It is purposed in these papers to bring out as fulh^ as possible such 

 interesting gradations, the present paper, of course, being confined to 

 the Argulidee. 



