36 Journal of Comparative Neurology. 



therefore in the more exposed parts of the body probably re- 

 ceive stimuli for the most part through direct contact with ex- 

 ternal objects. Those situated in the body epidermis can prob- 

 ably, if necessary, function in the same way, but it seems more 

 probable, since the surfaces in which they are situated are more 

 or less protected by appendages and since their sense-hairs 

 project but little above the general level of the body, that their 

 function is to receive tactile, or possibly chemical stimuli, 

 through the medium of the water. Nereis, when it leaves its 

 burrow, is greedily eaten by various other marine animals. The 

 motions of such animals in its neighborhood and perhaps the 

 chemical substences thrown off by them may be conveyed to it 

 through the water in advance of the animal and may thus en- 

 able Nereis to escape the threatened danger. The sense-organs 

 found in the buccal cavity probably serve as gustatory organs. 



The method by which mechanical stimuli are conveyed to 

 the cells of the diffuse sense-organs appears to be as follows : 

 If a living cirrus be watched under the microscope, it can be 

 seen that, when a foreign body comes in contact with a sensory 

 hair, the latter is bent passively to one side. This movement 

 may be conveyed to the nerve-cell in one of two ways. Either 

 the passive movement seen in the sensory hair is not confined 

 to this part alone, thus mechanically stimulating the nerve-cell 

 itself ; or else the movement of the sensory hair causes a me- 

 chanical irritation of the end of the peripheral process on which 

 it is borne, and this impression is conveyed to its cell by the 

 protoplasm of the process as a nerve impulse — that is, each 

 peripheral process is a nerve-fiber. The first method would 

 necessitate a straight, stiff structure capable of purely passive 

 movement. A peripheral process is always so delicate in struc- 

 ture and those in the cephalic cirri are so long and usually reach 

 the cuticula by such an indirect course that it seems to be a 

 mechanical impossibility for it to be a purely passive agent in 

 the conveyance of a stimulus. It would, therefore, appear that 

 each peripheral process, both from its structure, which exactly 

 resembles that of the central processes, and from its probable 



