THE HABITS AMD STRUCTURE OF AREX1COLA MARIXA. 109 



is a secretion of the walls of the otocyst, mixed with a little sea water. 

 The sand-grains are covered with a distinct layer of some chitinoid 

 substance soluble in boiling potash. Acids have no appreciable effect 

 upon these grains, and under the polariscope they react as quartz does. 

 Hence it seems clear that the otoliths of Arenicola marina (the other 

 species of the genus differ most remarkably in this respect, as well as 

 amongst themselves) are quartz grains covered by an organic film, and 

 surrounded by a fluid which is not merely sea water. 



Large specimens of the "Laminarian" variety were examined without 

 being opened under sea water, and the otocysts were mounted by us 

 in coslomic fluid. No movement of the otoliths was observed even in 

 specimens which were perfectly healthy in all respects. The otoliths 

 sometimes filled the expanded part of the organ, and it is possible 

 that they had no room to turn round. But it appears to us more 

 likely that if we assume the cause of the rotation to be the diffusion 

 caused by liquids so different as sea water, in which the preparation 

 was first mounted, and the somewhat viscous, perhaps albuminous fluid 

 inside the otocyst ; then if we mount the otocysts in the same kind of 

 fluid which they contain, no movement should occur; and the experi- 

 ment showed that in these cases no movement did occur. The whole 

 matter is one of very great interest, especially in view of the probable 

 functions of such an organ as the octocyst. Ehlers has suggested 

 that the movement is due to the cilia at the bottom of the neck of 

 the otocyst ; but the same extraordinary movements are seen in the 

 otocyst of A. Grubii, which is closed and has no cilia. We qrute 

 agree with Ehlers that there are no cilia in the expanded part 

 of the otocyst where the movement has been noticed, but we are of 

 the opinion that the quivering motion of the otoliths is not a normal 

 phenomenon, but is due to diffusion currents. 



9. Nephridia. 



There are six pairs of nephridia, belonging to somites 4 to 9. Of 

 these the first pair seems to be unrepresented in any other species of 

 Arenicola, and its variation in A. marina points clearly to a gradual 

 degeneration which it appears to be imdergoing at the present time. 

 It is not only the smallest of the series, but is sometimes represented 

 merely by a funnel or by the secretory and terminal portions. Very 



