ANATOMY AND CLASSIFICATION OF THE ARENICOLIDiE. 501 



ecaudata, not examined by him, possesses, according to 

 Fauvel (1899), otoliths of an entirely different nature from 

 those of A. Grubii, and also because we do not agree with 

 Ehlers that there is an open pit in A. Claparedii which 

 represents the earliest stage in the development of the organ. 

 The otocysts are a pair of vesicles derived from the skin. 

 In the common lugworm they open on the surface of the 

 body just behind the prostomium : more strictly speaking, on 

 the upper surface of the peristomial segment. In three other 

 species (A. cristata, A. Grubii, A. ecaudata) the vesicles 

 are closed, but they lie exactly below the point at which, in 

 A. marina, they open to the exterior. Finally, in A. 

 Claparedii there is no sense-organ, but Ehlers claims to 

 have shown that there is a pair of pits at the corresponding 

 points on the peristomium. We have examined several 

 series of sections of Naples specimens of this form, and we 

 find (PI. 27, fig. 56, xx) that there is an enlargement of the 

 metastomial groove at its point of origin behind the pro- 

 stomium. This enlargement in sections, taken in the same 

 plane (horizontal) as those figured by Ehlers, occupies the 

 site of the pit, which he refers to as marking the position of 

 the otocyst of other forms. In these sections the metastomial 

 groove is conspicuous on each side from the dorsal to the 

 ventral surface of the animal. The position of the otocyst (in 

 those species which possess the organ) lies slightly dorsal to 

 the point of intersection of the second transverse groove 

 behind the nuchal organ with this metastomial groove. This 

 point lies dorsally and to the inner side of the metastomial 

 pit of A. Claparedii, but Ave find no trace of any depression 

 in this species which can be accurately localised with the 

 point of origin of the otocyst. Ehlers states that the pit to 

 which he refers as marking this site is in no way modified ; 

 there is no special sensory epithelium, and no special oto- 

 cystic nerve. After careful examination we have concluded 

 that there is in this species no trace of the otocyst, nor is it 

 possible to localise the position it occupies in other forms 

 by any particular diverticulum of the metastomial grooves. 



