870 The American Naturalist. [September, 
The hooks depress the soft membrane between the coxopodite and 
basipodite on the dorsal-lateral aspect and catch firmly against the 
chitinous ridge formed by the hinge-like union of the chitinous edges 
of those same segments, coxopodite and basipodite. By this means the 
two animals are held together against the force necessary to introduce 
the male pleopods into the resistant annulus. 
The animals now remain united for several hours, during which time 
sperm is transferred into the annulus or seminal receptacle of the fe- 
male. 
The annulus is a well known descriptive character found in the fe- 
males of Cambarus, but not in Astacus: hitherto its use has not been 
known. 
It varies in shape in different species. 
In C. affinis its development varies, but in general it isa transversely 
elongated, ellipsoidal, chitinous elevation on the ventral side of the 
thorax between the bases of the fifth pair of walking legs. On this 
raised area are smaller, more prominent rounded elevations, bounding 
a transverse groove or pit. One of these is a gentle transverse ridge, 
forming the posterior lips of the groove; the other two are rather prom- 
inent bosses on the anterior lip of the groove. 
Between these last is a longitudinal cleft on the middle line, opening 
posteriorly into the transverse groove, and not straight, but curved as 
it passes between the two bosses. Sections of this organ show that the 
longitudinal cleft leads into a small pouch or sac that, when seen from 
a dorsal view, projects upward into the body as a curved ridge. This 
sac has firm walls that are of calcified chitin and presents no discov- 
ered opening except the external slit. It is regarded as simply a pit- 
ting in of the chitinous exoskeleton. 
After conjugation has taken place the annulus of the female has pro- 
jecting from its groove a small plug of whitish substance that may re- 
main for many weeks, 
The same material fills the cavity of the sac in the annulus. It isa 
compact, paste-like substance forming a tubular sheath around a cen- 
tral axis or mass of granules that on examination prove to be the pe- 
culiar, radiated sperm-cells of the crayfish. 
As the crayfish may be roughly handled and removed from one dish 
to another during the process of conjugation there is no difficulty in 
observing with a lens the means by which this sperm-plug is made. At 
this period of sexual excitement the terminal part of the vas deferens of 
the male is turned outward from the opening at the base of the fifth 
walking leg of each side and projects horizontally as a short, bent, con- 
