Langdon, Sense-organs of Nereis "uirens. 1 1 



peripheral processes of the cells of the same organ, to the base 

 of the cuticula covering the organ in question. Each peripheral 

 process presents exactly the appearance of a central one. There- 

 fore, as far as morphological appearances go, it is necessary to 

 consider each peripheral process as well as each central one a 

 true nerve-fiber ; as will be seen later, a functional interpreta- 

 tion tends to confirm this opinion. When the peripheral pro- 

 cesses from an organ reach the cuticula, they enter, still in a 

 bundle, the differentiated area which always exists in the cuticula 

 immediately over each sense-organ. 



The cuticula over the body of Nereis virens is composed of a thin outer 

 and a very thick inner layer (Plate II, Figs. 30 and 34). The outer layer is only 

 2 // thick and, as seen in sections, is clearly marked by straight striations perpen- 

 dicular to its outer surface. These striations mark the lines of separation between 

 the individual fibers of which this layer is composed. The inner layer isi8 // thick 

 and is faintly marked by undulating striations parallel to the outer surface. In 

 such stains as Kleinenberg's haematoxylin and Biondi-Ehrlich, the outer layer 

 takes a much deeper stain than the inner so that the plane separating the two 

 can be clearly seen. Bayer ('98) has found in the Rhynchobdellids that the cuti- 

 cula is composed of layers of different ages, that the striations in the cuticula 

 which run parallel to its outer surface represent the planes of separation be 

 tween these layers, and that the outermost layer is frequently shed. I know of 

 no observations on the shedding of the cuticula in the lower Annelids but the 

 wear to which the cuticula is subjected by external forces, the strain to which 

 it is subjected by the growth and movement of the worm itself, and also the 

 fact that the outermost layer is so easily detached from the rest of the cuticula 

 in both Nereis and Lumbricus "render it probable, it seems to me, that in the 

 lower Annelids the outer layer of the cuticula is either occasionally shed or 

 gradually Worn away. Then the undulating striations in the thick inner layer 

 of the cuticula of Nereis would represent planes separating the successive layers. 

 The breaking up of the outermost layer into fibers may be due to the weaken- 

 ,ing of this layer with age. 



The above mentioned differentiated area in the cuticula 

 over each sense-organ consists of two cavities — a larger ovoid 

 cavity in the inner layer of the cuticula and a shallow cavity in 

 the outer layer — separated by a thin layer which is perforated 

 by several fine canals (Plate II, Fig. 34 ; Plate I, Fig. 

 9). Each of these ovid cavities is 16 or 17 /< deep. A cross 

 section of it taken at any part of its height always presents a 

 circular outUne. A longitudinal section shows that the smaller 

 end of the cavity is next to the epidermis and is about 4 /^ in 



