NO. 1788. NORTH AMERICAN ERGASILID^— WILSON. 269 



being recognized for that sex one can not refuse it to the females also. 

 Indeed the present author has repeatedly witnessed it when the gills 

 of some fresh-water fish were placed under the dissecting microscope 

 for the purpose of removing these parasites. On being disturbed they 

 may often be seen to move considerable distances up or down the 

 filament to which they are clinging. This movement is accom- 

 plished by alternate motions of the second antennae and swimming 

 legs, in much the same way as a man uses his arms and legs in climb- 

 ing a pole. 



If there is this freedom of motion on a single filament it is reason- 

 able to believe that they can also move from one filament to another, 

 since the filaments are closely interwoven in the living gill. This 

 belief is strengthened by the fact that the parasites are usually found 

 on those filaments which are nearer the ends of the arches, leaving 

 the ones along the center free. This could hardly happen by chance 

 in so great a majority of the cases, but must be the result of a selec- 

 tive choice, which necessitates the power of moving about over the 

 gills in order to reach these positions. Many species of the genus 

 Bomolochus are found in other places upon the fish's body besides the 

 gills. And while in the water they are found capable of moving 

 about over such surfaces with the same scuttling motion shown by 

 Caligus, though not with the same rapidity. 



This family of copepods therefore possess all the methods of loco- 

 motion known in the group. 



Because they retain so fully their powers of locomotion the Erga- 

 silidse do not show as much degeneration as the Caligidse. Indeed 

 there is but a single genus, Tucca, which can be regarded as at all de- 

 generate. The others only exhibit the preparatory stage to degener- 

 ation in which the female becomes fixed upon her host and loses her 

 incentive for free swimming. 



Hosts. — In general these parasites may be found upon our common 

 food and game fish, often in considerable numbers. Among fresh- 

 water fish the bass, perch, sunfish, pike, and carp are nearly always 

 infested. This is especially true of the red-eye or rock bass, Amhlo- 

 plites rupestris, nearly every specimen of which is sure to yield para- 

 sites the number of which from a single fish often reaches the 

 hundreds. 



Among salt-water fish the needle fishes (EsocidaB), the gizzard 

 shads (Dorosomidae), the sauries (wScomberesocidse), the balaos (Hemi- 

 rhamphidae), the soles (Soleidae), and many members of the large 

 family of Gadidae are common hosts. 



Besides these which may be considered regular hosts, there are of 

 course many others upon which some species of the family may be 

 occasionally found. Owing to the fact that the great majority of 

 them retain so fully their power of free swimming, these Ergasilidae 



V 



