PKENTISS : THE OTOCYST OF DECAPOD CKUSTACEA. 205 



1. Structure of the Otocyst. 



a. Sac. The basal segment of the antennule in Carcinus is relatively 

 ]ar<'-e, and elongated laterally to such au 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 chitiu of the anterior part 

 of the segment {lab. 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 (lab. p.) is much lighter in color than 

 that in front. 



If the antennule of a crab is examined directly after ecdysis, when the 

 chitin is still very thin, soft, and uncalcilied, 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 line pair of forceps, 

 a transverse aperture is disclosed leading down to the lumen of the sac. 

 This aperture extends from line 45 (Fig. 4G) 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 tliat 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 measui'ements taken of the otocyst in speci- 

 mens approximating 30 mm. in length, gave the following results : — 

 Greatest length, 1.11 ram. 

 " width, 1.96 " 

 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 



