182 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
hypodermal cells, groups of cells with elongated nuclei; these send 
their processes into the bases of the bristles (Plate 2, Figs. 6,7). They 
are the matrix cells, which nourish the hair and, as we shall see later, 
have to do with its formation. The central region beneath the cushion 
is occupied posteriorly by the ganglion cells of the otocyst nerve (Plate 
2, Fig. 6, cl. gn.), and anteriorly by their peripheral fibres. 
c. Structure of hairs. The hairs of the otocyst are peculiarly modi- 
fied. Instead of being straight, as in tactile hairs, the shaft is here bent 
out of its course about 120°, so that its distal portion makes a sharp 
angle with the proximal end (Plate 2, Fig. 8). The shaft is very long 
in comparison with its diameter, being from 1604 to 200 u in length, 
while only 3 to 6 in diameter at the base. The part of it above the 
bend becomes extremely attenuate, and is heavily fringed with long deli- 
cate projections (pinnules), which give it the appearance of a plume. 
These fine feathery tips, which always project toward the concave side 
of the horseshoe formed by their bases, are crisscrossed and tangled to- 
gether in such a way as to form a wickerlike mesh, on which the majority 
of the otoliths rest (Plate 1, Fig. 3). The hairs are not attached firmly 
or immovably to the wall of the sensory cushion, but an exceedingly 
thin-walled chitinous bulb intervenes between the shaft and the wall of 
the sac. This, the spherical membrane of Hensen, is from 6 to 12 in 
diameter, and allows the shaft, itself comparatively rigid, to sway freely 
on its base, as if articulated there (Plate 2, Fig. 8, mb. sph.). 
d. The formation of hairs has already been described by Hensen 
(63, p. 374) in some detail. The conditions just before ecdysis were 
figured, but the earlier stages were not given; so a few supplementary 
facts may be added here.- Braun (’75) verified Hensen’s account of 
Haarwechsel in the bristles of Astacus, and himself discovered some 
new details. 
As before stated, each sensory hair is produced by a number of 
matrix cells, which send their processes into the shaft. In newly formed 
hairs, these protoplasmic processes extend to the very tip of the hair 
cavity (Plate 2, Fig. 7). In preparation for the next moult they are 
withdrawn nearly to the base of the hair, leeving the greater part of the 
hair cavity empty (Plate 2, Fig. 9). At the same time the matrix cells 
from which these processes are given off sink deeper into the tissue, below 
the level of the hypodermis, and with other chitinogenous cells originat- 
ing in the hypodermis, arrange themselves about the nerve fibre of the 
