PRENTISS: THE OTOCYST OF DECAPOD CKU3TACEA. 197 



It is thus relatively wider, and more shallow thaa that of Palaemonetes. 

 The wall is of thiu chitin continuous at the large oval aperture (Plate 7, 

 Fig. 30) with that of the dorsal side of the auteunule. The aperture is 

 as wide and nearly as long as the sac itself; instead of a fold of chitin 

 it has for protection a row of large fringed bristles. These are ranged 

 close together along the posterior edge of the opening and extend their 

 long parallel shafts beyond its anterior margin. A fine-meshed grating 

 is thus formed, through which even microscopic organisms could not pass 

 without displacement of the bristles. 



b. The sensory cushion (Plate 6, Fig. 29, crs. sns.), as already noted, 

 projects from the posterior portion of the lateral wall of the sac. Its 

 direction is not transverse to the long axis of the sac, but it points 

 obliquely forward and mediad. It is a ridge rather than a cushion, for 

 the hairs are arranged in a short, nearly straight single row, instead of 

 in several rows having the form of a sickle. This row of hairs, which 

 defines the limits of the sensory region, starting at the dorsal end of the 

 ridge, takes a course along its convex surface downward and backward, 

 and ends where the ridge disappears, just before the floor of the sac is 

 reached. A portion of a row of hairs is shown in the right otocyst, 

 Figure 29, set. ot. (Plate 6), where the hairs anterior in position are really 

 above or dorsal to those posterior to them. The ridge-like projection of 

 the sensory prominence is best seen in a parasagittal section (Plate 7, 

 Fig. 30, set. ot.), a hair being there shown at the apex of the ridge. 



The matrix cells are essentially the same as in the hairs of 

 Palcemonetes. They occupy the region just beneath the bristles, into 

 which their processes extend. The space in the sensory prominence 

 below and lateral to the matrix cells is occupied by the sensory ganglion 

 cells, the fibres from which penetrate between the formative cells and 

 reach the bases of the hairs (Fig. 29, cl. gn). 



c. Structure of hairs. Arranged on the sensory ridge in the manner 

 above described, the hairs of the otocyst are 26 in number, as shown by 

 the average of a lai'ge number of individuals. They are largest at the 

 upper anterior end of the row, where they measure 180 /« in length and 

 about 9 ju in diameter at the base of the shaft. Proceeding down the 

 line they are successively smaller, the last of the series being only 1 00 fi 

 in length and 6 ^ in diameter. There is a conspicuous spherical enlarge- 

 ment at the base of the hair shaft (Plate 7, Fig. 31, mb. sph.), as in the 

 otocyst hairs of Palaemonetes. The shaft itself for about a third of its 

 length projects straight out horizontally into the lumen of the sac. 

 Then it bends down ventrally nearly at right angles, though the amount 



VOL. XXXVI. — KG. 7 3 



