PRENTISS : THE OTOCYST OF DECAPOD CRUSTACEA. 173 



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 Palsemon. 



Innervation of the Otocyst. In Palsemou 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 (" Chordae " 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 

 langere Strecke weit bis zu einem Horhaare hinfort, und geht durch die 

 Mitte des Porenkanals imd der Haarkugel bis zur Lingula hin, an die er 

 sich festsetzt." Essentially the same conditions were found by Hensen 

 in Carcinus maenas and in Mysis. He also found nerve fibres supplying 

 the tactile bristles which are present on all pai'ts 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. Tin's 

 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. 



