12 Journal of Comparative Neurology. 



diameter ; the diameter increases gradually until, at the peri- 

 pheral end of the cavity, it becomes about 8 n . This ovoid 

 cavity does not extend entirely through the inner layer of the 

 cuticula, but is bounded peripherally by a thin layer of the 

 latter — a layer which forms the perforated membrane previously 

 mentioned. The membrane is about 2 jn thick, is slightly con- 

 cave on both sides, and is pierced by from 5 to i6 minute canals. 

 Over it is the cavity in the outer thinner layer of the cuticula. 

 This cavity is really an opening which apparently extends en- 

 tirely through the outer layer of the cuticula. This opening, 

 together with the slight concavity in the outer side of the per- 

 forated membrane itself, forms on the outer surface of the cuti- 

 cula over each sense-organ a saucer shaped depression slightly 

 more than 2 m in depth and about 6 or S m in diameter (Plate 

 I, Fig. II). 



The entire bundle of peripheral processes from a single 

 sense-organ enters the ovoid cavity of its own differentiated area 

 and passes through this cavity to the perforated membrane. 

 Here the peripheral processes terminate in Sensory hairs which 

 pass through the canals in this membrane into the outer cuticu- 

 lar cavity and in the latter form a brush-like group which 

 projects stiffly above the external surface of the body. Usually 

 each peripheral process bears a single sensory hair which passes 

 directly through one of the canals in the perforated membrane 

 to the exterior. In a number of cases, however, I could dis- 

 tinctly see that before passing to the exterior a peripheral pro- 

 cess branched into at least two or three processes (Plate II, Fig. 

 37), thus bearing several sensory hairs, each of which passed 

 through a canal of its own. Owing to the difficuly of obtain- 

 ing a side view of a cluster of sensory hairs in the removed 

 epidermis, I found it impossible to study those of the general 

 epidermis in the living condition. In material killed by alcohol 

 or Miiller's fluid and stained by the usual stains, not only are 

 the cuticular canals but dimly seen, but the sensory hairs are 

 usually withdrawn so that their tips lie either within the canals 

 of the perforated membrane or even beneath the latter in the 

 ovoid cavity itself. I am, therefore, unable to state how far 



