PKENTISS: THE OTOCYST OF DECAPOD CKUSTACEA. 211 



which, in the crab, with tlie disappearance of the otoUths, 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 Carciuus. 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 110/^ to 135 /^ long and \2 fi to 

 14juin 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 

 Palaemonetes. From the presence of a cup-like depression at the 

 base of each shaft, instead of the lar-ge spherical membrane found in 

 the Macrura, it might be inferred that the cup results from the in- 

 complete evagiuation 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 



