NO. 2194. NORTH AMERICAN PARASITIC COPEFODS— WILSON. 11 



upon end in a circle so that the long axis of the parasite will, if pro- 

 duced, pass through the center, and at right angles to the plane of 

 the circle. Radii are then drawn through the center of the anal and 

 mouth apertures and projected upon the plane of the circle. The 

 angle between these radii is the angle of torsion and is expressed 

 in degrees. There are almost always longitudinal ridges or stripes 

 or something of the sort which indicate clearly the direction of tor- 

 sion, whether to the right or left. 



So far as observations have gone there seems to be considerable 

 regularity in the torsion, not only as to the direction, but also as to 

 the amount. Furthermore if the torsion of any given species is direct 

 on the right side of the host it is more often inverse on the left side, 

 and the opposite. But the present author is by no means prepared 

 to go as far as Quidor does and say that the kind and amount of 

 torsion possesses specific value. On the contrary both prudence and 

 actual observation strongly prohibit any such conclusion. In view 

 of the extremely meager data at present available it seems presumptu- 

 ous to assume that two given specimens of any genus are distinct 

 because they were found on the same side of their host, and the one 

 showed direct torsion while in the other it was inverse. And the dis- 

 tinction between a direct torsion of 45° in one specimen and 90° in 

 another possesses still less value. 



Subsequent observations may show that such distinctions ought 

 to be considered in connection with certain genera, but the following 

 tables prove conclusively that in the genus Lemaeemcvs they are not 

 worth considering. 



Sixteen tomcod heads were preserved by Mr. V. N. Edwards at 

 Woods Hole, each of which was badly infested on the side of the 

 head, along the margin of the gill covers, and behind the pectoral 

 fins with Lernaeenicus affixus^ the number of specimens varying from 

 2 to 24: and the total reaching 138. The direction and amount of 

 torsion of each parasite was carefully noted and is recorded in the 

 tables, together with the position on the host. 



It is assumed that in its normal position the dorso-ventral axis of 

 the parasite is at right angles to the surface of the host's body and 

 that the ventral surface of the parasite is next to the skin of the 

 fish. This position is indicated in the tables by the term " dorsal 0°," 

 and shows that there has been no torsion. 'WTien the parasite has 

 been turned 180° in either direction its dorsal surface will be next 

 to the skin of the fish, and this position is indicated by the term 

 " ventral 180°." In case of doubt as to the direction of torsion the 

 entire parasite was removed. The tomcod heads are numbered con- 

 secutively, the numbers appearing in the first column of the tables. 



