AMERICA N PARASITIC ARG ULID^^ WILSON. 643 



These are wholly external parasites, and though sometimes found 

 upon the skin or the tins, they are usually confined to the branchial 

 cavity of their host. They may be sought on the inner wall of the 

 operculum or in the shallow pocket behind the posterior gill arch, but 

 are never found upon the gills themselves except through accident. 

 They cling to their host by means of the anterior maxillipeds which 

 are modified for this purpose into sucking disks, somewhat similar to 

 those upon the arms of Cephalopods. And by a sort of walking motion 

 of these same suckers they are enabled to scuttle about quite rapidly 

 over the fish's skin so long as it remains moist. But they are unable 

 either to fasten themselves to, or to make any progress over, a dry sur- 

 face. The posterior maxillipeds are also modified into clasping organs 

 armed with hooks at the tips, spines, and setw along the sides, and a 

 large plate on the basal joint whose surface is usuall}^ raised into rough 

 papilla?, and whose posterior edge is furnished with three stout spines 

 whose chief use seems to be to act as a firm brace while the pointed 

 proboscis is being thrust forward into the flesh of the host in order to 

 draw blood. And finally the basal joints of the anterior antennae are 

 developed into a pair of enormous curved hooks which assist somewhat 

 in holding the Argulus to its host. But their chief use is apparently 

 to keep the anterior edge of the carapace firmly in position while the 

 proboscis is being worked. 



Whether the Argulus remain upon the outer skin or in the branchial 

 cavity it is continually subjected to considerable friction as the fish 

 moves about through the water, especially if its host happens to be a 

 fish of rapid movements, and there is an absolute certainty that it will 

 be washed off if its hold is loosened for ever so short an interval. 



To lessen the friction as well as to get the full use of its grasping 

 organs, the Argulus (and the same is also true of the Caligidte) always 

 takes a position with its own longitudinal axis parallel to that of its 

 host, and its head pointing- in the same direction. In this position all 

 the grasping organs just mentioned work together to prevent an}- slid- 

 ing l)ackward over the fish's skin. In addition, the lower surface of 

 the carapace, more particularly at the anterior margin and along the 

 edges, is quite thickly studded with short triangular spines, which 

 point downward and backward, and catch firml}- in the skin of the 

 host. At the bases of both pairs of antennte, and in many species, 

 just behind the mouth and between the bases of the second maxillipeds 

 are paired spines uuich larger and stouter, which evidently serve a 

 similar purpose. 



Though there may be little evidence of degeneration in these Argu- 

 lids, therefore, there are abundant modifications to suit their acquired 

 parasitic habits. 



Upon the death of its host an Argulus leaves the body at once if it 

 be in the water and swims about actively in search of a new victim. 



