ANATOMY AND CLASSIFICATION OF THE ARENIC0LID2E. 508 



cliitinoid otoliths appear to be correlated witli a closed 

 vesicle. 



We are unable to give a detailed account of tlie structure 

 of the wall of the otocyst in this species, as our specimen 

 was not sufficiently well preserved. The cells of the wall 

 are of two kinds, — columnar supporting cells and fusiform 

 sense cells; and the cavity of the vesicle possesses a dis- 

 tinct cuticular lining (PL 27, fig. 65). The nerve-supply to 

 the organ is derived from the oesophageal connectives. 



In A. ecaudata and A. Grubii the two otocysts (PI. 27, 

 fig. 64) have the same relative position (peristomial) as in the 

 two former species. They are closed vesicles, into the base 

 of which a sheet of muscle derived from the body-wall is 

 inserted, as Ehlers has described in detail for A. Grubii. 

 The long columnar cells forming the wall of the otocyst are 

 of two kinds — sense cells and supporting cells. The former 

 end in one or more delicate points which project through the 

 cuticular lining into the fluid with which the vesicle is filled. 

 Around the basal ends of these columnar and sense cells, there 

 is a slight amount of connective tissue, and in it is embedded 

 a plexus of nerve-fibrils — the termination of the nerve to the 

 otocyst, — which enters the organ at its outer and under side 

 and spreads out over its surface (Fig. 64, N. 8h.). The special 

 development of ganglion cells on the course of this nerve and 

 at its point of origin from the connective is described in con- 

 nection with the nervous system (p. 479). 



The otoliths of perfectly fresh specimens of both these 

 species, often, but not always, exhibit the peculiar rotatory 

 movement which was first described by Quatrefages in the 

 otocyst of the common lugworm. It is best seen if the 

 otocyst is rapidly dissected out under sea water. If the 

 lugworms are not quite healthy the movement of the otoliths 

 does not occur, and these cases of immobile otoliths are 

 probably due to some alterations in the fluid inside the 

 otocyst. We have examined the lining of the vesicle very 

 carefully without detecting auy trace of cilia, and we are 

 inclined to consider the movement due to diffusion currents. 



