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are withdrawn, the matrix cells recede from the base of the old hair, and 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 tlie 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. 



{h) 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 antennas, and ventral to the optic centres. The 

 nerve sheaths disappear as the fibres enter the " Punktsubstanz," and 

 the fibrillse 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 fibrillse, 

 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 tlie base of the hair 

 which they supply, and show no fibrillar structure. 



12. In the shrimp-like decapods, such as Palsemonetes and Crangon, 

 a nucleated myelin sheath surrounds each sensory fibre and ganglion 



