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 prin¬ 
cipal modifications are these: 
a. Form and size of the spines of the first basal segments. 
b. Position of the lateral hairs of the second basal segments. 
c. Relative 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. 
f. 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 may 
be one- or two-segmented, may be rudimentary, or may acquire 
considerable length. Its tip may be armed with setae, or with 
two more or less prominent spines. 
PRIMITIVE STRUCTURAL CHARACTERS. 
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 difficulty 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 
