Langdon, Se7ise-organs of Nereis virens. g 



above each organ and project above the external surface as a 

 group of sensory hairs. These sense-organs are directly com- 

 parable with the epidermal sense organ of Lumbricus (Hesse 

 '95 and Langdon '95) and of Axiothea and Clymene (Lewis 

 '98). The sense-organs found in different regions of the body 

 vary in the position of their cells, in the final termination of 

 their .peripheral processes, and in the character of the cuticular 

 openings by means of which these processes reach the exterior. 

 The organs of these various regions will, therefore, be described 

 separately. Since those found in the epidermis of the body 

 itself may be conveniently taken as a type, they will be first 

 considered. 



A. Structtire. 



a. Diffuse sense-organs of the body epidermis. — Each dif- 

 fuse sense-organ of the body epidermis, as seen in the prosto- 

 mium, peristome and first metamere, consists of from 5 to 8, 

 rarely as many as 16, bipolar nerve-cells. These cells are ar- 

 ranged in a slender spindle-shaped group varying from 4 to 10 

 i-i in width and from 16 to 20 /< in length (Plate II, Figs. 30-32) 

 and all of the cells in each group are situated entirely within the 

 epidermis itself Each of these bipolar nerve-cells may be con- 

 veniently analyzed into three parts — the body of the cell, an 

 enlarged part in which the nucleus lies, the central process or 

 axis cylinder, a slender fiber-like part which arises from the cen- 

 tral end of the cell-body, and the peripheral process, an equally 

 slender part which arises from the peripheral end of the cell- 

 body. The tip of this peripheral process is differentiated into 

 a sensory hair which is raised above the external surface of the 

 worm (Plate I, Fig. 9 ; Plate II, Fig. 34). 



The body of an individual cell is about 4 n wide and from 

 4 to 10 /< long, scarcely larger than its nucleus. The shorter 

 cell-bodies round off abruptly at each end ; the longer ones 

 taper at one or both ends. From the central end of each cell, 

 one and only one process arises. I have never found any divis- 

 ion of this process nor any branches arising from it, but the 

 cases in which I have been able to trace a central process until it 



