NO. 2194. NORTH AMEltWAN PARASITIC VUl'EPUDS—M'ILSON. 7 



Consequently the only dimorphism would be one of length and not 

 of structure, a difference which is common to all copepods, but is 

 here carried a little farther than usual. 



Locomotion. — Judging by the size one would say that the larvae 

 alone possess the power of locomotion, and that the adults are fixed 

 parasites. 



But we must remember that sexual union takes place during the 

 first parasitic period, and that it is followed by a second period of 

 free swimming during which both the male nnd female must be 

 sexually mature. 



Moreover, the male never develops any farther, so that he is as 

 fully matured as he will ever become, and the subsequent develop- 

 ment of the female is a retrograde metamorphosis rather than an 

 evolution. Accordingly, we are justified in saying that both the 

 larvae and the sexually mature adults are capable of free swimming, 

 and in proof of this both are frequently taken in tow. The majority 

 of males remain attached to their host after sexual union, but all 

 the females and a small percentage of the males (4 per cent) dis- 

 engage themselves and become free swimmers for a second time. 

 Each of these free-swimming periods is fully as long as that of the 

 Lernaeopod larva, so that the two together give the Lernaeid larva 

 twice as much free swimming as is enjoyed by the Lernaeopodidae. 



In the nauplius and metanauplius larvae tha swimming motion is 

 jerky and more or less spiral in direction; in the copepodid larvae 

 it becomes faster and more continuous, although it still remains 

 somewhat spiral. 



These larvae dart about by spasmodic contractions of the swim- 

 ming legs and search for something to which they can fasten them- 

 selves. If they find nothing suitable they swim about until tired, 

 come to rest upon their backs on the bottom of the aquarium, and 

 remain thus for long periods. 



Prehension. — On seeking its first or temporary host the larva uses 

 its second antennae and maxillipeds for prehension. After settling 

 upon the gills in this manner it either continues to use these append- 

 ages like Lernaea^ or fastens itself by a broad frontal filament to the 

 tip of the gill filament like Lemaeocera., Sarcotretes. and Pennella. 

 In the former case no change is produced in the host's gills, but in 

 the latter case the attachment of the parasite's frontal filament to 

 the fish's gill causes the gill to swell up and become tumid, while the 

 filamentous plates disappear on both sides for some distance from 

 the tip. 



One or the other of these two modes of prehension, or both of 

 them, are the only ones ever practiced by the male; but the female 

 when she searches out a final host adopts a third method. She bur- 



