CONJUGATION IN THE CRAYFISH, CAMBARUS AFFINIS 255 
The way in which these strong hooks are used needs to be 
more clearly shown. The bases of the third legs are lifted up and 
away from one another by the male when mounted upon the 
female and then brought toward one another till, like spurs, the 
hooks fit into the joint between the second and third segments 
of the female's legs. This can be easily done, manually with 
dead or living crayfish, if only the legs of the female be held up 
against the sides of the thorax. The joint between the second 
and third segments of the legs is a soft membrane that can be 
displaced and the hard hook shoves it in. The face of the hook 
then comes against the hard rim of the second segment. The 
rim of the joint shows a small tubercle both on the first and the 
second segment. When the limb is extended these hinge tubercle 
are side by side, but when the limb is flexed up against the body 
the two tubercles together make a stiff, semi-circular rim about 
the soft membrane. It makes as it were the hollow of an elbow 
into which the hook enters and out of which it cannot come 
sidewise because the above hard rim opposes the flattened con- 
cave, or proximal, face of the hook. The overhang of the tip of 
the hook (fig. 4) seems advantageous. The semi-circular pit of 
the female's leg exists only when the leg is bent. Hence, when 
we hook two dead specimens together the weight of the female 
may be held up by the male hooks, mechanically, without nms- 
cular effort, as long as the limbs have the general positions shown 
in fig. 2; but if the leg of the male is raised, or if the leg of the 
female be straightened out the hook is loosened and the female 
drops off. 
The mechanical nature of this use of the hooks is, poorly, 
indicated in the section, fig. 2. This shows, however, the inverted 
female containing ovarian eggs and the male with the coils of 
efferent ducts in like region of the body, poised upon the female. 
The bases of the limbs only come into contact, while the ventral 
surfaces of the bodies of the animals are separated by the space 
caused by the concavity of the ventral surface of the thorax of 
the male especially. It is across this space that the stylets will 
be extended to reach the annulus, which is indicated on the middle 
of the upturned surface of the female. The hooks on the legs of 
