390 Wisconsin Academy of Sciences, Arts, and Letters. 



5. The form of the fifth feet of the male. In the male fifth 

 feet, there are two basal segments. In the right foot, the ex- 

 opodite consists of two segments, the second segment bearing a 

 lateral spine and a terminal hook. The endopodite is one- or 

 two-segmented. In the left foot the exopodite is composed of 

 two segments, the second segment terminated with two proc- 

 esses. The endopodite is one- or two-segmented. The pria- 

 cipal modifications are these : 



a. Form and size of the spines of the first basal segments. 



h. Position of the lateral hairs of the second basal segments. 



c. Eelative lengths of the segments of the exopodite. 



d. Position of the lateral spine of the second segment of the 

 exopodite. 



e. Form and size of the terminal hook. 



/. Form and size of the processes and lamellae which some- 

 times occur on the segments of the right foot. 



g. Form of terminal processes of second segment of the ex- 

 opodite of the left foot. 



h. Form, size and segmentation of the endopodite. It maj 

 be one- or two-segmented, may be rudimentary, or may acquire 

 considerable length. Its tip may be armed with setae, or witk 

 two more or less prominent spines. 



PBIMITIVE STRDCTUKAL CHABACTERS. 



In discussing the structural relationship of the species, it is 

 necessary, if any phylogenetic conclusions are to be reached, 

 to determine what are the more primitive characters. This is 

 a matter of some difiiculty and must be largely, perhaps, con- 

 jecture. 



The typical copepod appendage consists of two basal segments 

 with three-segmented exopodite and endopodite. We may as- 

 sume that the fifth feet of both sexes have been derived from 

 such a typical structure by a process of reduction. I call this 

 an assumption, for I do not feel certain that it is true. It 

 seems, however, most probable in the light of present knowl- 

 edge. Granting this assumption, it would follow that the most 

 primitive form would be the one that most nearly approaches 



