PKENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 195 



still further. There is a deep lateral, as well as a posterior, fold in the 

 chitiu ; but the sac, if it can now be called such, is very shallow, wide- 

 mouthed, and without sensory hairs or otoliths. From the group of 

 matrix cells, however, the tips of embryonic sensory hairs 'may be made 

 out, projecting dorsally, but covered by the chitinous floor of the sac 

 (Plate 5, Fig. 27). Only after the wall of the sac has been shed at the 

 next moult will they become functional organs. 



d. Fourth Larval Stage. 



(Form like that of adult ; thoracic exopods rudimentary.) 



The sac has now greatly increased in size, and nearly hlls the cavity 

 of the appendage (Figs. 24, 25). Its opening has become smaller, 

 and is protected by numerous fringed bristles, which project from its 

 sides (Fig. 25, tct.). About 200 sensory hairs are present borne on a 

 prominent sensory ridge (Fig. 24, set. ot.) and arranged in three regular 

 rows, one row less than in the adult stage (Fig. 26). The whole band 

 bears some resemblance to a sickle. Beginning at the median side of the 

 sac floor, the rows curving only slightly run laterally, then with a 

 stronger bend turn forward. At tlie anterior end of the sac regular 

 arrangement ceases, the hairs being grouped promiscuously. Besides 

 these large hairs on the sensory ridge, which measure 120ju to 150/z in 

 length and from 4^ to 6 ^u in diameter, there is, as in adults, an irregular 

 row of more attenuate hairs arranged longitudinally along the posterior 

 part of the median wall (set. m., Fig. 26). They number about thirty, 

 are on the average 140 fi in length, and have a diameter of only 2fito3(x 

 at the base of the shaft. 



Many otoliths, consisting of fine particles of sand, rest on the hairs of 

 the sensory ridge, as in the adult condition, but do not come into con- 

 tact with the attenuate bristles of the median side-wall, which project 

 free into the liquid contents of the otocyst. The sensory ridge is much 

 more prominent at this stage than in the adult. This, and the size of 

 the aperture, are the chief differences between the two, and are well shown 

 m Figure 25. The opening gradually becomes smaller in the fifth, sixth, 

 and seventh stages, until in the full-grown animal it is almost obliterated. 

 A fourth row of hairs, not yet developed, is formed posterior to the 

 others at some stage later than the seventh moult, this being the oldest 

 stage that I have studied. Except for the gradual closure of the aper- 

 ture, the larvPB of the fifth, sixth, and seventh stages show the same 

 conditions in the otocyst as the stage under consideration. 



In Figure 24 (Plate 5) ganglion cells (d. gn.) are seen beneath the 



