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E. A. ANDREWS 
directly concerned in sperm transfer for another publication. 
These hooks are mere hard tubercles, or blunt spines on the shell 
of the third segment of each third leg, (counting the chelate legs as 
the first of a series of five legs) fig. 3. 
Many other kinds of crayfish have none, others have one on the 
second as well as on the third leg, while others have one on the 
fourth as well as the third leg. 
The hook is thus not an organ absolutely necessary to crayfish 
conjugation, though we shall attempt to show that it has become 
necessary in C. affinis. 
Fig. 3 Posterior face of left third leg of male 64 mm. long. 
In the female there is no hook and in the male there is no simi- 
lar protuberance on the other legs, so that in C. affinis the organ 
is not metamerically repeated, while the other cases referred to 
show that it may be repeated. Moreover the spine is not a speci- 
alization of something found on other legs in that region but is 
in each case a perfect spine or nothing. To be sure there are simi- 
lar spines on the big claws and on the body, but not in the region 
corresponding to the hook. 
In C. affinis the hook or spur (fig. 3) makes a very conspicuous 
projection from the proximal part of the third segment of the leg, 
forming a strong cone with its proximal face flattened, and sparsely 
set with setae. The whole is white and bony with the tip espe- 
cially so and as if rolled over as a terminal ridge (fig. 4) . 
Springing from the underside the spur juts downward and also 
much toward the body and is so long that in old individuals 
(fig. 5) it runs far across the liiie of joint between the second and 
third segment and tends to become more parallel to the leg. 
