254 
E. A. ANDREWS 
There is thus with age an increase in the size and efficiency of 
the hook, for the more the tubercle becomes recu;:"ved the more is 
it fit to be a hook that will hold strongly to the female as will 
be seen presently. In old hooks, fig. 5, the base extends nearly 
the whole length of the third segment of the leg. 
Sections, such as fig. 6, show that the hook is hollow; that is, 
it is an outgrowth of the shell, lined by epidermis and filled with 
connective tissue, as are the large tubercles elsewhere on the 
shell. The hook is thus a special employment of a local outgrowth 
just as the claw is a special use of the opposition of the terminal 
segment of the leg to a like tubercle, -or outgrowth that opposes 
the last segment. 
In the development of the male C. affinis, the hooks come quite 
late and are perfected after they have been used. 
Crayfish of this species reared from eggs laid in the spring and 
measuring in October 22 mm., had no hooks (fig. 7), but merely 
the appearance of a false joint parallel to the real joint between 
segments two and three and a row of plumose setae in the region 
of the future hook. 
But in males of 48 mm. of the same date (fig. 8) there is a 
large rounded lump with some of the above setae upon it. The 
section of this specimen (fig. 6) shows the thickness of the shell 
that continues over the spur and also the arrangement within the 
second and third segments of the muscles which thus have no 
connection with the region of the hook. 
In the outgrowth of the spur the large setae are left about its 
base and some small ones are developed over its proximal face. 
These are well seen in living specimens with shells not too much 
worn, as in the specimen of 64 mm. shown in fig. 4. In many 
specimens, as in fig. 5, the setae are broken off more or less. These 
setae are long enough to suggest that they serve as sense hairs 
or as tactile setae, which may be of use in enabling the male to 
adjust the hook to its socket. The short ones along the proximal 
face may also serve pressure sense. 
The above male (fig. 3 and fig. 4), being killed February 1st, 
had doubtless used its hooks in conjugation, as that has been 
found to take place in the first fall when the males are but 50-60 
mm. long. 
