NO. 1404. PARASITIC COPEPODS—CALIGID.E—WILSOK 483 



her eo-o-.s aljout witli her like most of the copepods. This habit neces- 

 sitates several departures from the conditions found existing among 

 the Arg'ulida'. 



In the first phice we must look for a greater ditference between the 

 sexes ]»oth in their morphology and in their habits. 



The genital segment of the female is considerably larger than that 

 of the male, and usually the first antenna' are larger and stouter. On 

 the other hand, the second maxillipeds are larger in the male, and the 

 abdomen is often composed of two segments, while the female has 

 only one. The increase in the genital segment of the female, together 

 with the hea\y egg cases which she has to carry, restrict her freedom 

 of motion. 



And hence while both sexes can swim about freel}' it is only the 

 males which can be expected to compare fav^orabl}^ with the Argulidte 

 in this regard. This sex difi'erence is particularly emphasized during 

 the breeding season or just at the time when there is the greatest incen- 

 tive for free swinuning. That this restriction of the female's motion 

 is at least favorable to degeneration can not be dou))ted. But at the 

 same time we have to remendjer that all the copepods save the Argu- 

 lidaj are burdened in the same way, and 3'et all free-swinmiing forms 

 are able to combat the condition successfully. The condition in itself, 

 therefore, is scarcely enough to be regarded as the first step toward 

 degeneration; we must seek something more. 



In ordinary free-swimming forms the female, even when burdened 

 with her egg strings, must move about in search of food. In fact, she 

 needs food more then than at any other time. 



Again, in the Argulidas, the female deposits her eggs upon some con- 

 venient surface away from the })ody of her host, and such deposition 

 becomes not an incentive merely, but an imperati\'e demand for free 

 swimming. The males follow the females at these times and also 

 search for them from fish to fish. 



CaJlgus females, on the contrary (and the same applies to all para- 

 sitic genera), carr3dng their eggs about until they hatch, find the sui- 

 face of the fish's body one of the ])est possible positions to secure good 

 aeration for the eggs and to discharge the nau})lii when they are suffi- 

 ciently matured. 



Finally these parasites feed upon the blood of their host, or at least 

 upon something which they obtain while upon the host's l)()dy; hence 

 ])y remaining here they are nearest the source of their food supply. 

 In short, all the incentives are for remaining, rather than leaving the 

 host and swinmiing about, and adult females almost always remain 

 upon the fish, even during the periods when they are without egg- 

 strings. 



The only inducement in these forms to free swimming on the part 

 of the female would be that which is common to all parasites, plant 



