PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 195 
still further. There is a deep lateral, as well as a posterior, fold in the 
chitin ; 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 fills 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 the anterior end of the sac regular 
arrangement ceases, the hairs being grouped promiscnously. Besides 
these large hairs on the sensory ridge, which measure 120 uw to 150 uw in 
length and from 4 u to 6 uw 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 w in length, and have a diameter of only 2 to 3 u 
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 
in 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 larvee 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 (el. gn.) are seen beneath the 
