484 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvin. 



and animal alike, namely, the original search for a suitable host. But 

 this operates in developmental rather than in adult stages, and it is a 

 significant fact in this connection that nearly every female of these 

 genera which has been captured in the tow has been immature. 



The mechanical hindrance afforded b}^ the egg-strings, together with 

 the strong incentives just enumerated for remaining upon the body of 

 the host, may be fairly considered as constituting the first step toward 

 degeneration. Let us now look at the mode of locomotion in these 

 genera in order to discover the second step. 



LOCOMOTION. 



There are two modes of locomotion as in the Argulidee, a free- 

 swimming and a scuttling motion. The presence or absence of the 

 latter has a greater significance than has hitherto been accorded to it. 

 By watching specimens of Argulus and Caligus in an aquarium it can 

 be seen that the latter are really the better swimmers. This is due 

 to the increased surface of the first three pairs of legs, particularly to 

 the large lamina or apron which connects the third pair across the ven- 

 tral surface of the body. These legs furnish a swimming organ which 

 propels the copepod through the water with strong and swift move- 

 ments. Often the motion is so erratic and persistent that the animals 

 seem to have fairly gone mad, dashing frantically about, turning sum- 

 mersaults, rushing for a distance along the sides of the aquarium, or 

 scuttling back downward across the under side of the surface film of 

 the water. Equipped with such a swimming apparatus it would nat- 

 urally be supposed that they would put it to frequent use, but we have 

 already seen that they lack the incentive. As a matter of fact, mature 

 females of but one or two species have ever been taken with the males 

 at the surface. 



Furthermore, as will appear in the descriptions which follow, these 

 few specimens were all of the genus Caligus, the species rapax being- 

 most commonly secured. The other genera here treated do not show 

 as much inclination to free swimming as Caligus and there are but one 

 or two very doubtful records of their capture in the tow. Indeed, 

 A. Scott goes so far as to conclude from a series of careful investiga- 

 tions that " L&peojphtTieirus throughout the remainder of its life and 

 under normal conditions remains on the same fish that it attached 

 itself to at the beginning of the chalimus stage." And the same might 

 probably be said with regard to Anuretes and Trebius. 



When we consider the amount of surface towing conducted every 

 year under the auspices of the United States Bureau of Fisheries, the 

 scarcity of these parasitic forms can only mean that at least the mature 

 females are not accustomed to swim freely at the surface, but only do 

 so under extreme provocation. 



