AMERICAN PARASITIC ARGULID.E— WILSON. 675 



Reproductive organs are present and so far developed that the sex 

 of the larva can be told at birth. 



5. In the second group, which requires two months or over for 

 incubation, the development of the larva inside the egg is carried still 

 farther, so that it emerges not merely with all its appendages present, 

 but with each full}' developed, except the anterior maxillipeds, and 

 all (even these) performing their appropriate functions. The swim- 

 ming legs are sufficiently developed to l>e used exclusively for locomo- 

 tion; the second antennae are similar to those of the adult, and there are 

 no temporary mandibular palps. The first maxillipeds and the rudi- 

 mentary sexual organs are similar to those in the other group. 



In these larv». therefore, there is no real metamorphosis after birth, 

 since nothing but the first maxillipeds are at all changed in structure and 

 even these serve the same function from the first. 



6. By a series of molts at intervals of a few days the larva is 

 gradually transformed into the adult. At the fourth molt, sixteen 

 days after hatching, the sucking disks are so fully developed as to 

 begin to function, and the sexual organs have matured, so that this 

 molt is to be regarded as the close of the larval period. 



MORPHOLOGY. 



Body regions. — In consequence of the peculiar relations existing 

 between the parts of the body in the Argulidae we find different authors 

 assigning entire!}^ difi'erent names to them. This has been due in 

 large measure to those modifications of the paired appendages resultant 

 upon the parasitic habits of the animals, e. g., the hooks upon the ante- 

 rior antenna?, the sucking disks upon the anterior maxillipeds, etc. 



These modifications caused numerous mistakes in the early interpre- 

 tations of the appendages, with corresponding mistakes in the body 

 regions to which they are attached. 



To Thorell (18()4) and Claus (1875) more, perhaps, than to any other 

 investigators are we indel)ted for the elimination of these errors, the 

 scientific comparison of this group with others closely related to it, 

 and its final establishment in its present position. 



But both these authors worked with a very limited number of 

 species. Claus's observations were confined to the two fresh-water 

 si)ecies foliaceus and coregoni, and Thorell onl}- added one other form, 

 purpureux., which, however, was a parasite on salt-water fish. In view 

 of such a meager supply of species upon which to base family charac- 

 teristics, and the consequent doubt as to how fully they might have 

 represented the entire family, it is hoped that the following compara- 

 tive study of 13 American species, together with the comparative 

 ontogeny which has preceded, may serve to supplement the data 

 already given, and so strengthen Claus's assignment of the group. 



The body of an Argulid divides itself naturally into three regions — 



