ORGANS FOR SPERM-TRANSFER 247 
that the appendage has very little mobility back and forth 
through some 45°. The normal position of the stylet is pointed 
forward under the thorax, where it lies horizontally in a deep 
groove, but in use it is dropped down and backward toward a 
vertical position. It has an anterior face, which is usually carried 
as the dorsal side, a posterior face which is usually the ventral 
aspect, and an outer and an inner or median face. 
The general appearance of the stylet is seen in the photographs, 
figs. I, II, 111,1v, which represent respectively the posterior, median 
(or rather median and posterior somewhat diagonally), the anter- 
ior and the outer faces of the same left stylet. Fig. 1, the posterior 
face, is the view got by looking at the under side of the crayfish, 
after lifting up the second stylet, which les over the first and 
largely conceals it. 
The first pair of stylets do not spring from the sternal surface 
far apart as is the case with the common, unmodified swimmeret, 
but they arise very close together; in fact the median faces, (fig. 
11,) of the two come into contact so that these two appendages really 
form one mass. If looked at from the dorsal side, the two are 
seen to lie in contact at the base and all along the distal half, 
leaving between the constricted parts of the two a square opening 
that is occupied, in rest, by part of the second stylet. 
In describing the stylet we will distinguish the base, the neck, 
and the scroll or spiral that contains the groove. The scroll ends 
in two tips, the more slender, side outgrowth, or spatula, and the 
real end bearing the groove, the canula 
The base is some 6 mm. wide and long and only 2 thick, being 
flattened from before back. ‘The posterior or ventral face of the 
base, fig. I, presents a wide groove bounded on the median side by 
a rounded knob and on the outer side by a long ridge which, as it 
passes on to the neck, bears a tuft of long, finely plumose setae, 
that are seen again in profile in figs. i, iv. In this deep groove 
the second stylet lies when not in use, so that the two appendages 
are firmly packed together under the thorax of the male. 
The part of the base joined to the sternum of the animal is an 
oblique elliptical area, around the edge of which the hard shell 
gives place to the soft articular membrane that makes it possible 
