PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 243 
are withdrawn, the matrix cells recede from the base of the old hair, and 
arrange themselves about the nerve fibre for the formation of the new 
bristle. There is a period between moults, more or less extended, dur- 
ing which no living substance is present in the greater part of the cavity 
of the hair. 
6. The otoliths are grains of sand taken in from the exterior (first, in 
the case of the lobster, by the fourth larva) and renewed after each 
moult ; they may lie free in the otocyst, or be attached to the sensory 
hairs. In Brachyura they are found only in the Megalops stage. 
7. Glands similar in structure to the tegumentary glands are present 
in the lobster and crayfish beneath the sensory cushions which bear oto- 
lith hairs. They secrete a substance for the attachment of the otoliths to 
the pinnules of the bristles. 
8. The innervation of the otocyst hairs and olfactory bristles is dis- 
tinctly unlike. 
(a) The otocyst hairs have each a single nerve element, and the 
terminal fibre ends in the enlarged base of the shaft without branching. 
(6) Each olfactory bristle is innervated by numerous ganglion cells 
(100 or more). The peripheral strand of fibres from these cells extends 
some distance into the cavity of the hair, terminating free and without 
modification of any kind. 
9. The central terminations of all the otocyst fibres are in two closely 
connected neuropilar masses at the posterior end of the brain, median 
to those of the second antennz, and ventral to the optic centres. The 
nerve sheaths disappear as the fibres enter the “ Punktsubstanz,” and 
the fibrilla soon separate. They cannot be traced to determinate 
endings, nor are they ever directly connected centrally, with ganglion 
cells. 
10. Each sensory nerve fibre is composed of numerous fibrille, 
embedded in a semi-fluid ‘‘ perifibrillar” substance, which in turn is 
surrounded by a delicate sheath. The flowing together of the peri- 
fibrillar matrix causes the beaded or varicose appearances characteristic 
of methylen blue, and silver impregnations. The fibrillar structure can 
be demonstrated in both the central and peripheral portions of the 
fibres. 
11. The sensory ganglion cells are all typically bipolar and elongate 
in form. They are placed at some distance from the base of the hair 
which they supply, and show no fibrillar structure. 
12. In the shrimp-like decapods, such as Paleemonetes and Crangon, 
a nucleated myelin sheath surrounds each sensory fibre and ganglion 
