200 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
3. Development of the Otocyst. 
This was not studied in Crangon. 
II]. CamBaRUS AFFINIS (Say) GIRARD. 
The otocyst of the crayfish has been figured by only Farre (’43) and 
Huxley (80). The description of the former investigator was excellent 
for the time at which it was made. Huxley alludes to the otocyst in 
his work on the crayfish, and gives one figure showing the sensory 
region dissected out. Hensen (’63) describes the hairs of the otocyst 
in Astacus fluviatalis, but does not touch upon its other structures. 
1. Structure of the Otocyst. 
a, Sac. The otocyst of Cambarus (Plate 8, Figs. 37, 38), except for 
its smaller size, resembles that of the lobster very closely. The aper- 
ture, exceedingly small in the lobster, is here quite large, though, on 
account of the dense chevaux de frise of fringed bristles, it seems smaller 
than it really is. These bristles, projecting from around its margin, 
effectually cover and conceal the opening. It occupies the middle of the 
dorsal side of the antennule; its anterior margin corresponds to the 
anterior wall of the otocyst, and it extends back from this point nearly 
one-half the length of the sac. Its width is about one-third that of the 
otocyst (Fig. 37). 
The cyst does not by any means fill the cavity of the antennule. It 
is rounded off in front, but sharply pointed at its posterior end, where 
it is very shallow (Fig. 38). Its walls are of uncalcified chitin and 
continuous with the very thick calcified shell of the antennule (Figs. 37, 
38). Its dimensions in average-sized animals are: 
Length from 1.75 mm. to 2.25 mm. 
Wadithy Se" Wh 52! Be eee Z STO Ke 
Depth: 4/0852 AGRE 4 
b. Sensory Cushion. The sensory ridge, or cushion, in the base of 
the otocyst is not prominent, as that part of the sac floor upon which 
the sensory hairs are borne is but slightly elevated above the rest (Fig. 
38, set. ot.), and, contrary to the conditions found in the two forms already 
described, the sensory surface is nearly horizontal, instead of being 
vertical or oblique. The arrangement of the hairs is shown in Figure 
40 (Plate 8). Three sets can be distinguished, corresponding to the 
divisions of the otic nerve, —a median, a lateral, and a transverse or 
posterior. The first and third are nearly straight, the second sickle- 
