PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 197 
It is thus relatively wider, and more shallow than that of Paleemonetes. 
The wall is of thin chitin continuous at the large oval aperture (Plate 7, 
Fig. 30) with that of the dorsal side of the antennule. 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, ers. 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 
Paleemonetes. 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, el. 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 large number of individuals. They are largest at the 
upper anterior end of the row, where they measure 180 w in length and 
about 9 in diameter at the base of the shaft. Proceeding down the 
line they are successively smaller, the last of the series being only 100 u 
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 Palemonetes. 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 
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