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, 

 tct), 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 apertni'e of the otocyst, while its forward projection over the an- 

 terior lip of the slit can be seen in Figure 5 (Plate 2) at tct. 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. 



6. 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 cristce acusticce 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 gi'adually 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 layer of 



VOL. XXXVI. — NO. 7 2 



