no. 1404. PARASITIC COPEPODS—CALIGIDM-WILSON. 4^5 



Such a change in habits, constituting- as it does a long step toward 

 that fixedness of position which precedes radical degeneration, must 

 have some adequate cause. These three genera have practically the 

 same swimming apparatus as Caligus, and if it is never used there 

 must be some preventive influence which operates in their case but not 

 in that of CoMgus. 



In the author's opinion this influence may be found in the presence 

 of sucking disks on the frontal plates of 'Galigus and their entire 

 absence in the other genera. Their presence gives to Caligus the 

 same scuttling motion as Argulus obtains from its first maxillipeds. 

 In this way they move about over the surface of their hosts with great 

 rapidity and upon the slightest provocation. But the other genera, 

 lacking the sucking disks, are dependent upon the second antennas and 

 the maxillipeds for locomotion over the surface, and can not conse- 

 quent^ move about with any rapidity. For this reason they do not 

 change their position as often as Caligus but remain a long time fixed 

 in one place. Indeed, when an attempt is made to remove them from 

 their host, only the males and immature females move about in order 

 to escape. The mature females usually settle down in situ and only 

 cling the more tightly. When removed from their hosts and pi iced in 

 aquaria these genera settle upon the bottom or sides and remain sta- 

 tionary for long periods, in marked contrast to the restless activity of 

 Caligus. This fixit}^ of position can not help reacting unfavorably 

 upon any tendency toward free swimming which might still be 

 retained by the copepod. 



To recapitulate, then, we find that none of the Argulida exhibit 

 degeneration or even any tendency toward it. They have all retained 

 completely both the ability and the inclination to swim freely and to 

 move about over the body of their host. 



Among the Caliginre the genus Caligus possesses even more ability 

 than Argulus, and the males and immature females retain practically 

 the same incentives. But for the mature females every influence oper- 

 ates toward remaining upon their host, and they are very seldom cap- 

 tured swimming freely. All the species of Caligus, however, still 

 move about over their host's body upon the slightest provocation. 

 Other genera, being destitute of lunules, lack the ability to move 

 about on the body of their host with any freedom. 



This acts as a still stronger damper upon their movements, and 

 although they retain fully the ability to swim they almost never exer- 

 cise it. They not only remain upon one host all their lives, but they 

 :ilso fasten themselves in a single spot and stay there continuously. 

 They thus exhibit the initial stages of degeneration, whose next step 

 is to be a partial loss of the ability left unused. 



While speaking of locomotion mention must be made of a pernicious 

 habit common to many of the Caligime. This consists in crawling up 



