PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. PAIL 
which, in the crab, with the disappearance of the otoliths, have taken 
on the chief functional activity of the otocyst, formerly vested in the 
hook hairs. 
(3) The group hairs (set.’) form the third and most numerous class of 
the otocyst bristles of Carcinus. Irregularly distributed in the most 
lateral corner of the sac (Fig. A,) on a flattened portion of the wall 
ventral to the closed margins of the aperture (Plate 9, Figs. 42, 47; 
Plate 10, Fig. 55), they are unlike any of the otocyst hairs found in 
Macrura, being short, thick, and blunt, without a trace of fringing fila- 
ments (Plate 10, Fig. 49). They are 1104 to 135 long and 124 to 
14 in diameter. There are nearly 200 of these hairs, forming one 
large irregular group. They do not occur in the Megalops otocyst, 
therefore they must be developed at some later period. They may 
possibly be degenerated tactile hairs which in the formation of the 
otocyst have been folded into its cavity. Their proximity to the aper- 
ture of the otocyst makes this supposition highly probable. Their 
shafts are set into depressions in the sac wall, and, like the other oto- 
cyst hairs, they can sway freely on their bases. 
d. Formation of Hairs. The hairs are formed in Carcinus, and in 
the Brachyura generally, after the method already described in 
Palemonetes. From the presence of a cup-like depression at the 
base of each shaft, instead of the large spherical membrane found in 
the Macrura, it might be inferred that the cup results from the in- 
complete evagination of the hair. 
e. Otoliths are entirely wanting in the adult otocyst, but are present 
in those larval stages where the sac is still open. They consist, as 
usual,.of grains of sand, which in this case are very small, for the sac 
itself in these stages is less than 0.3 mm. in length. They can readily 
be introduced into the otocyst of the Megalops, as its aperture is rela- 
tively large. When in a succeeding stage the sac is cast off with its 
otoliths at ecdysis, the aperture of the new cyst closes at once, and 
no foreign particles can enter it ; henceforth it is without otoliths. 
2. Innervation of the Otocyst. 
The general course of the otocyst nerve is shown in Plate 10, Figure 
55 (n. ot.). As in the forms previously described, the sac lies in close 
proximity to the brain, and its nerve is consequently short. It is 
given off with the antennular nerve from the anterior end of the cen- 
tral organ, and its course for a short distance is directly lateral, until 
the base of the antennule is reached. At this point the antennular 
