ORGANS FOR SPERM-TRANSFER 265 
by a very stiff oblique joint at about 45° and allows of very little 
lateral and rotative motion. It may be forced outward and in- 
ward through but few degrees, its tip traveling only 4 mm. _ It 
may be twisted so that the triangle, from being almost concealed 
dorsal to the end of the bony mass (fig. v), may be turned outward 
a few degrees toward a horizontal position and present more of 
its median face, somewhat as in fig. v1. The movement is com- 
parable to that of a stiff arm that should allow only a little side- 
wise movement and a very little twisting at the elbow with the 
end result that the triangle, or hand, at the end, accomplishes a 
little adjustment to the orifice of the first stylet. This is done as 
if by supination, though done by the above twist at the elbow. 
The flagellum is the real termination of the endopodite; it is 
some 3 mm. long, 1 mm. wide and rapidly tapering, also flattened, 
being a long triangular terminal tip to the essentially flat endopo- 
dite. By the presence of white lateral areas in the otherwise mem- 
branous flagellum, it is obscurely divided into 9 or 12 joints. At 
the tip and along the sides it bears long plumose setae that are 
often sparse or worn off along the outer side. The flagellum 
springs from a socket in the bony shell of the wide end of the endo- 
podite. The external angle of the edge of this socket, figs. v 
and vi, forms a hard protuberance at the end of a bony ridge 
(the Guide). The setae along the flagellum as well as those 
along the exopodite do not stand out horizontally, right and left, 
but slant ventrally, or posteriorly, (fig. vir). 
The most novel and characteristic part of the second appendage 
of male crayfishes is the lateral outgrowth which we will call the 
triangle. It is a form of the Decapod appendix masculina of 
Boas. The triangle stands up dorsally so that at rest, it, with its 
fellow of the other side of the body, fits into the squarish cavity 
left between the two necks of the first stylets. It is not well 
seen normally from the ventral view, (fig. v) but it may be pulled 
outwards through 90° and then looks as in the median view (fig. 
v1). It isa flat triangular outgrowth, partly calcified and partly 
membranous. The edges are calcified and the centre membran- 
ous, so that the whole suggests a bent arm or wing with skin 
stretched across it. Each long side of the triangle is about 3 and 
