no. 1404. PARASITIC COPEPODS— CALIGID2E— WILSON. 483 



her eggs about with her like most of the copepods. This habit neces- 

 sitates several departures from the conditions found existing among 

 the Argulidag. 



In the first place we must look for a greater difference between the 

 sexes both 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 antennae 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 heavy egg cases which she has to carry, restrict her freedom 

 of motion. 



And hence while both sexes can swim about freely it is only the 

 males which can be expected to compare favorably with the Argulidas 

 in this regard. This sex difference is particularly emphasized during 

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

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

 is at least favorable to degeneration can not be doubted. But at the 

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

 lidse are burdened in the same way, and yet all free-swimming 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 Argulida?, the female deposits her eggs upon some con- 

 venient surface away from the body of her host, and such deposition 

 becomes not an incentive merely, but an imperative demand for free 

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

 search for them from fish to fish. 



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

 sitic genera), carrying their eggs about until they hatch, find the sur- 

 face of the fish's body one of the best possible positions to secure good 

 aeration for the eggs and to discharge the nauplii 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 body; hence 

 1>\ remaining here they are nearest the source of their food suppty. 

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

 host and swimming about, and adidt 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 



