10 PROCEEDINaa OF THE NATIONAL MUSEUM. vol.63. 



ceplialotliorax begin to grow, since it would be manifestly impossible 

 to move a set of long branched horns thrc)iigh flesh. 



Torsion. — Baudouin (1905Z*), Quidor (1912), and some other ob- 

 servers have called attention to the torsion or twisting of the body of 

 the adult females, which may be due to several causes. The anterior 

 portion of the parasite's body is buried in the tissues of the host, and 

 is held rigidly in place by the horns and lateral processes of the 

 cephalothorax, and by the tough cyst that forms around them. The 

 posterior portion of the body hangs freely in the water and can be 

 moved in any direction. 



"When the parasite takes a position on its final host we may assume 

 that its own long axis is parallel with that of the host (external 

 parasites), or with the water current (gill parasites). The body of 

 these female parasites is greatly elongated, while the antennae and 

 mouth parts are close to the anterior end. Consequently when the 

 parasite attaches itself to the fish and begins to burrow its way into 

 the flesh the long body will trail backAvard in the water in such a 

 way as to offer the least resistance. Furthermore the fish's scales 

 are arranged in such a way that if the external parasite is to burrow 

 under them, as is often the case, it must assume this position. At 

 first the posterior portion of the body would have a tendency to 

 stand out from the surface of the fish, but it is quickly bent by the 

 resistance of the water and thus the body of the parasite comes to 

 have a curve or bend just where it leaves the body of the host. The 

 angle of this bend varies with the direction taken by the burrowing 

 copepod. If the latter bores its way into the host at right angles to 

 the surface (Sarcofretes) the bend in its bod}'^ will be approxi- 

 mately a right angle; but if the parasite bores in obliquely the an- 

 gle will be larger (Lernaeenicus) ; and in the case of very oblique 

 boring there may be no bend at all {Lernaea^ species). 



In those parasites whose point of entrance is on the median line 

 of the host this simple flexion of the body seems to be the only result 

 produced. Specimens of all three of the genera mentioned above 

 have been taken by the author from the median line of various hosts, 

 and their bodies showed no torsion whatever. This fact seems to 

 have escaped the observation of Baudouin and Quidor; at least they 

 make no mention of it. But when, as usually occurs, the point of at- 

 tachment is on the right or the left side of the host, then there fol- 

 lows a torsion or twisting of the body of the parasite upon its long 

 axis in addition to the flexion. 



Of course the body may turn either to the left or the right, and 

 Quidor has defined as direct that torsion in which the body is turned 

 from the left over toward the right when viewed from the anterior 

 end, and as inverse when it turns from the right over toward the 

 left. The amoimt of torsion is measured by standing the parasite 



