PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 207 
of the posterior wall of the sac, bears a number of hairs with hooked 
shafts. The surface bearing these lies in a nearly vertical plane. From 
its position and the shape of its hairs this prominence is comparable 
to the sensory cushions upon the surfaces of which the otoliths are 
lodged in Paleemonetes, Crangon, and the crayfish. Irregularly disposed 
matrix cells are situated in clusters immediately beneath the hooked 
hairs (Plate 10, Fig. 50), and deeper in the tissues are the ganglion 
cells of the nerve fibres which supply the bristles (Fig. 50, e/. gn.). In 
the larval stages of the crab this sensory cushion is relatively much 
larger. It extends through half the length of the sac, and its hairs 
are in contact with the otoliths which the sac then contains. Pores of 
tegumental glands penetrate the chitin of this prominence, as they do 
that of the sensory cushions in the crayfish and lobster, although found 
in no other part of the sac. These glands secrete a substance which, in 
the larval crab, attaches the otoliths to the tips of the hairs. Their 
presence in the adult crab is evidence in favor of tht homology of this 
cushion with that described for otocysts containing otoliths. 
The other sensory cushion is much larger, and is produced by a 
partial invagination of a portion of the median and anterior walls of 
the sac, which forms an oval prominence (Fig. A; Plate 9, Fig. 48; 
Plate 10, Fig. 55, set. fil.). It is nearly 0.5 mm. in diameter, and its 
surface, making an angle of about 45 degrees with both the transverse 
and sagittal planes of the animal, inclines backward, inward and down- 
ward (Plate 9, Fig. 45). Its ventral portion is shown in transverse 
section in Figs. 47 48. The chitin of this cushion is very thin ; upon 
it is arow of long delicate hairs, called by Hensen (’63) “ Fadenhaare,” 
or thread-hairs. This row runs down somewhat obliquely from the 
upper side of the prominence to its ventral margin near the floor of the 
sac, its dorsal end being the more anterior of the two (Fig. A, set. jil.). 
This sensory cushion is also found in the sac of the larva, and 
the bristles it then bears are similar to those found projecting free into 
the lumen of the lobster otocyst from its median wall (Plate 5, Fig. 
26, sel. m.). The prominence we are now describing in Carcinus is 
probably therefore simply a further differentiation of the slight projec- 
tion noted in the sac of the lobster. 
Matrix cells send delicate processes into the hairs, as in those of pre- 
ceding species; the ganglion cells are situated directly beneath the 
hypodermis, but some distance posterior to the bases of the hairs (Plate 
10, Fig. 53, cl. gn.). 
No gland pores are present, nor are they needed, as the thread 
