PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 205 
1. Structure of the Otocyst. 
a. Sac. The basal segment of the antennule in Carcinus is relatively 
large, and elongated laterally to such an extent that its width is nearly 
twice its length (Plate 9, Fig. 46). Along its dorsal wall there ex- 
tends transversely a distinct line dividing the chitin of the anterior part 
of the segment (lad. a.) from that of the posterior. This line of division, 
which reaches from the lateral margin of the segment three-fourths of 
the way across its dorsal wall, is rendered more prominent from the fact 
that the chitin posterior to it (/ab. p.) is much lighter in color thau 
that in front. 
If the antennule of a crab is examined directly after ecdysis, when the 
chitin is still very thin, soft, and uncalcified, this lighter colored area 
(Fig. 46, lab. p.) is found to be a fold, projecting forward over the ante- 
rior part ; and if its edge is lifted with a needle or fine pair of forceps, 
a transverse aperture is disclosed leading down to the lumen of the sac. 
This aperture extends from line 45 (Fig. 46) laterally down through 
the side wall of the antennule. There is, then, in fact, a free passage 
into the otocyst directly after moulting, a condition necessitated by 
the casting off of the old sac. But almost immediately after ecdysis, 
the opening is closed and its edges fuse together, probably owing to the 
simultaneous secretion of chitin by the hypodermis of the two surfaces 
which bound the orifice and are in direct contact. Figure 44 (Plate 9) 
shows at lab. p. the two surfaces which fuse. 
The form of the sac is very irregular, so much so that Hensen de- 
spaired of describing it. Its walls, like those of the forms already studied, 
are continuous dorsally with the calcified chitin of the antennule (Figs. 
42-48, Plate 9). The sac is thus suspended from the dorsal wall of the 
appendage. Although composed largely of thin chitin, one portion of its 
wall is much thickened and calcified (mal., Figs. 43-48, Plate 9). On 
account of its irregular outline measurements can be of only small value. 
The average of a number of measurements taken of the otocyst in speci- 
mens approximating 30 mm. in length, gave the following results : — 
Greatest length, 1.11 mm. 
coe iwidth, 1296 
« depth, 1.05 “ 
The seemingly contorted shape of the sac is caused by three protuber- 
ances or invaginations of its walls, which project into the lumen (Fig. A, 
and Plate 10, Fig. 55). Two only of these prominences are sensory and 
bear bristles (Fig. A, set. ta. and set. fil.). The third and largest of 
