PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 17a 
Hensen also found on the appendages of some decapods free hairs 
which closely resembled auditory bristles, and are described as such by 
him. Crangon especially, which has few hairs in the otocyst, is supplied 
with many of these so-called “free auditory hairs.” They are also 
numerous in Mysis and Palzemon. 
Innervation of the Otocyst. In Palemon Hensen traced the nerve of 
the first antenna from the brain. A large branch of this nerve runs to 
the ventral side of the otocyst, where the fibres separate, each enlarging 
into a ganglionic cell and then proceeding to the base of a hair. Each 
of these terminal fibres (‘‘ Chorde ” according to Hensen) then enters 
the pore beneath a hair, passes through the spherical membrane to the 
lingula, or process from the base of the hair shaft, and makes itself fast 
to this. In his own words (Hensen, ’63, p. 368): ‘ Dieser eigenthiim- 
liche Faden, den wir als Chorda bezeichnen, lauft eine kiirzere oder 
lingere Strecke weit bis zu einem Hérhaare hinfort, und geht durch die 
Mitte des Porenkanals und der Haarkugel bis zur Lingula hin, an die er 
sich festsetzt.” Essentially the same conditions were found by Hensen 
in Carcinus menas and in Mysis. He also found nerve fibres supplying 
the tactile bristles which are present on all parts of the decapod body. 
Formation of New Hairs (Haarwechsel). New hairs are not formed 
inside the old, but beneath the chitinous wall ; and instead of developing 
from a single matrix cell, as was supposed, Hensen found that each was 
the product of a great number of cells. A new layer of chitin is 
formed beneath the old, and under this new layer, but continuous with 
it, the new hairs are formed as double-walled (i. e. invaginated) tubes. 
The new chitin wall is compared to the hand of a glove. If the 
fingers of the glove be turned partially outside in, so as to leave only 
their tips projecting, the condition would represent that of the hair 
tubes just. before the moulting of the old shell. The tips of the newly 
formed hairs become attached to the shaft of the old hair, into which 
they project some distance, and as the latter are detached at ecdysis, 
the new hairs are pulled out. Nerve fibres were found running into the 
very tips of the new hairs. Hensen’s theory is, that at moulting, the 
old nerve fibre, becoming more highly refractive and resembling chitin, 
is, upon the detachment of the old hair, drawn out through the apex of 
the new one, and that before this event a new fibre is formed. This 
theory, however, is not easily reconcilable with his statement that the 
nerve fibres attach themselves to the lingula at the base of the hair shaft. 
The remainder of this part of his paper is devoted to brief descriptions 
of the otocyst as found in some twenty-four different species of Crustacea. 
