Langdon, Sense-organs of Nereis virens. 5 i 



a side view, the ovoid part of the refractive bodies, often 

 does not show. (PI. II, Fig. 46). These parts are, how- 

 ever, so generally found in views from above and in macerations 

 that I am inclined to think they are always present ; probably 

 when apparently absent, they are concealed by a thick layer of 

 protoplasm. The different appearances presented by these re- 

 fractive bodies do not seem to be limited to any particular part 

 of a spiral organ or to the organs of a definite region of the 

 body. Occasionally I have seen in the gill lobes peripheral 

 processes which contained refractive bodies of abnormal size or 

 shape or even accessory bodies which were entirely unconnect- 

 ed with the apical one. (PI. II, Fig. 48). As far as I can 

 judge, these accessory bodies always have the same optical prop- 

 erties as the normal refractive bodies. 



The refractive bodies might be either a hardened secretion 

 of the cells with which they are connected or the metamor- 

 phosed tips of the peripheral processes. The protoplasm of 

 the peripheral process is usually found to entirely surround the 

 ovoid part of a refractory body and sometimes even the wedged- 

 shaped blocks. This shows that, whatever its method of for- 

 mation, a refractive body is an inclusion of the cell which pro- 

 duces it. 



Under the influence of macerating fluids, the peripheral 

 processes swell and soon go to pieces. The central tube not 

 only remains attached to the cuticula but the refractive bodies 

 remain firmly attached to the tube although component blocks 

 of the spiral band sometimes separate a little from one another. 

 It has therefore so far proved impossible to obtain for study a 

 perfect detached element of a spiral organ — i. e., a cell with all 

 its processes and its refractive body. The small size of the re- 

 fractive bodies and their peculiar spiral arrangement around a 

 tube of so small diameter produces many optical effects which 

 render difficult the study of these bodies. The foregoing ac- 

 count however contains only descriptions which have been 

 tested by repeated observations. 



The spiral band may be either a left- or a right-handed 

 spiral but the two never occur in the same organ. The en- 



