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 \Yhich the otoliths are 

 lodged in Palsemonetes, 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, cl. 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 the 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 a row 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, set. 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.). 



Xo gland pores are present, nor are they needed, as the thread 



