PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 181 
nule (Fig. 1), as its anterior end is more lateral in position than the 
posterior. The external aperture has the form of a pointed ellipse and 
penetrates the dorsal wall of the antennule ; it is nearly as long as the 
sac itself, but does not extend quite as far back as the sac. It was 
described by some of the early writers as a longitudinal slit, by others as 
transverse; but, as Hensen points out, it is neither: its direction is 
oblique, and corresponds to that of the long axis of the otocyst. The 
opening is completely covered over by a thin fold of chitin (Figs. 1, 3, 
tet.), which extends forward and laterad to end in a sharp projection or 
spine. This lid-like fold (tectum) must be lifted or cut away in order to 
come directly at the opening of the otocyst. Figure 3 shows the position 
and form of the lid in transverse section, and how closely it fits over 
the aperture of the otocyst, while its forward projection over the an- 
terior lip of the slit can be seen in Figure 5 (Plate 2) at tet. As the 
chitinous lining of the otocyst is of ectodermal origin, like all other 
chitinous parts, it is cast off at each ecdysis, with all it contains, and a 
newly secreted sac takes its place. 
b. The sensory cushion of the otocyst is produced by an elevation of 
the median and posterior portion of the floor of the sac, which projects 
into the lumen and gives a somewhat constricted appearance to the cyst 
in sagittal sections. The surface of the cushion, which is about 0.25 mm. 
in diameter, is not horizontal, but slants downward from the median 
side of the sac to its lateral wall at an angle of nearly 45° (Plate 1, 
Fig. 3). This makes the sensory cushions of the right and left sides per- 
pendicular to each other, a condition similar to that described for Mysis 
by Bethe (’95*, p. 556), and of some physiological importance. The 
sensory hairs are borne on the sensory cushion, and for this reason 
the prominence has been compared to the créste acustice of vertebrates. 
The hairs, or bristles (for both names are applied to them), vary from 
forty-five to fifty-eight in number, and are arranged in a curved horse- 
shoe-like row (Plate 1, Fig. 1), the two ends of which are directed 
obliquely caudad and mediad. Largest at the inner end of the curve, 
and arranged in a single row, they grow gradually smaller toward 
the other end of the series, where an irregular double line is formed. 
Fig. 6 (Plate 2), a transverse section through the posterior ends of the 
horseshoe shows the base of a single hair on the right or median side, 
while at the left or lateral end two bristles are seen, the lateral row 
being double. 
Directly beneath the hairs we find, instead of the usual iayer of 
VOL. XXXVI.— NO. 7 2 
