268 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



has shown that the best way to accompHsh this on the gills is as fol- 

 lows. The body of the parasite is always parallel with the gill fila- 

 ment around which are clasped either the second antennae or the 

 maxillipeds. The head of the parasite is always toward the base of 

 the filament or the gill arch from which the filament proceeds. If a 

 blunt pointed needle, like a tape needle, be inserted from the base of 

 the filament between the latter and the body of the parasite, the 

 creature may be swept down along the filament and off its end with 

 considerable ease. This method possesses the further advantage of 

 removing as little slime along with the parasite as possible and in par- 

 ticular, by holding the mouth-parts out away from the filament, helps 

 to keep them clean and free from obnoxious matter. 



Locomotion. — The two sexes are similar in structure ; each possesses 

 four pairs of well-developed swimming legs, which are perfectly capa- 

 ble of functioning during the larval stage of development. But while 

 this power is retained by the adult male, it is usually diminished in the 

 female, and the latter when placed in an aquarium often shows no 

 tendency to swim about, but lies inert upon her back wherever she 

 may be placed. Under sufficient provocation, however, the females 

 can move with considerable rapidity, but never with the agility 

 exhibited by the males. 



T. Scott, after noting that Bomolochus solex is frequently found in 

 the nostrils of the cod, adds (1901, p. 122) : 



This habit on the part of B. solese, is the more interesting when it is remembered that 

 it is a "free-living" species, and that there appears to be nothing to hinder it from 

 leaving the nostrils of the fish, for it can move freely about amongst the mucus with 

 which the nostrils are usually well supplied, and if the copepods are removed and 

 placed in clean sea water they may be seen swimming or running about with nearly aa 

 much agility as the free-swimming species. 



Both sexes of Ergasilus chautauquaensis were captured with other 

 Crustacea while swimming freely at the surface of Lake Chautauqua 

 in New York. And the males of all the common species of Ergasilus 

 are found in the tow of our fresh-water lakes and ponds as has been 

 noted by nearly every observer in recent years who has studied the 

 plankton of such localities at all carefully. These observations all 

 go to prove the close relationship of the present family with the 

 free-swimmers. They show that, in spite of the parasitic habits they 

 have acquired, they still retain to a considerable degree this form of 

 locomotion. 



But they have also acquired another method of moving about which 

 has come as a direct result of their parasitism. Vogt has called atten- 

 tion (1877, p. 98) to the fact that the two sexes are never found at- 

 tached side by side on the same gill filament, but the male is always 

 at some distance from the female. Hence it follows that the males 

 at least must move about in search of the females, and this ability 



