Langdon, Sense-organs of Nereis virens. 7 



cuticular location of each sense-organ is shown by a clear white spot 

 in the surrounding blue stained tissue. 



Owing to their minuteness, the external openings of the spiral or- 

 gans could not be distinguished in the removed cuticula. The distri- 

 bution of these organs in the head and first metamere was studied by- 

 means of a wax model of this region. In making this model it was 

 necessary, owing to lack of material, to make use of a series of sections 

 only ID /< thick and prepared for another purpose so that there were 

 no reference planes \ but there are so many grooves and ridges on the 

 surface of the anterior end of Nereis that it was easy, by their aid, to 

 fit the sections together accurately. As each organ appears in at least 

 two sections, only every other section was drawn and the position of 

 each organ was marked in its margin. After the sections were cut out 

 and as they were being fitted together, common pins were stuck into . 

 the model so that their heads marked the position of the outer ends of 

 the spiral organs on the surface of the model. 



III. External Appearance of Nereis virens. 



The following brief description is inserted for the convenience of 

 the reader. The body of Nereis virens is in general cylindrical, and 

 beside the head itself consists of about 120 metameres. The head 

 consists of a prostomium joined to the dorsal border of the anterior 

 margin of a wide peristome which is supposed to consist of two fused 

 metameres (see Text-figure i ). The prostomium bears the four eyes 



Text-figure i . Outline of the dorsal surface of the cephalic end of Nereis 

 virens. dr., cephalic cirrus; <?., eye ; w<?/., metamere; /«/., palp ; /ar., para- 

 podium ; per., peristome ; pros., prostomium ; ten., tentacle. 



on its dorsal surface, a pair of palps on its lateral border, and a pair of 

 tentacles on its anterior border. Each tentacle is a small appendage 

 which tapers to a point. Each palp consists of two parts — a thick basal 

 part and a smaller rounded tip which can be almost wholly retracted 

 into the basal part ; the body cavity extends into the basal part but not 

 into the tip. Just caudad of the base of each palp, the peristome bears 



