504 -p. W. GAMBLE AND J. H. ASHWORTH. 



although it has precisely the same appearance as the move- 

 ment of the otolith of certain Lamellibranchia, e. g. Cyclas, 

 which is caused by ciliary action. 



In post-larval A. ecaudata there is only a single minute 

 otolithj which floats in the viscous fluid with which the 

 vesicle is filled. The otolith is spherical, 7 ju in diameter, 

 and has a minute central granule. As the worm increases in 

 size this otolith becomes larger and others arise, but the first 

 one always remains conspicuous from its larger size. In full- 

 grown specimens the large otolith measures "036 to '04 mm. 

 in diameter, and the small ones '003 to "01 mm. They are 

 all spherical, somewhat yellowish, highly refractive bodies^ 

 with slight indications of concentric lamination. Each is 

 covered by a thin transparent envelope. In the centre of 

 each otolith there are usually a few dark granules. The 

 characters of the otoliths in A. Grubii agree precisely with 

 the foregoing description. 



The chemical reactions of the otoliths in A. Grubii have 

 been somewhat fully recorded by Ehlers, with whose state- 

 ments and results we are in full accord. " Ich halte," says 

 Ehlers, " die Substanz dieser Otolithen danach fiir eine, 

 welche dem Stoffe gleich kommt, der die Cuticula und die 

 Borsten bildet." The chief ground for this couclusion is the 

 behaviour of the otoliths on the addition of acids. Weak 

 hydrochloric and strong acetic acids have no effect. Strong 

 hydrochloric acid, however, causes the otoliths to swell, 

 become clear, and lose their original shape ; but while these 

 alterations are going on there is no trace of the evolution of 

 gas. We have obtained exactly the same result. Further, 

 on placing the otoliths in eau de Javelle they become more 

 transparent, especially if slightly warmed, and at the same 

 time lose their refringent character. The otoliths apparently 

 arise in the glandular cells of the otocyst wall, and are 

 discharged into the fluid filling the cavity when they are 

 still minute (1 /u or so in diameter). In the fluid they 

 gradually increase in size, but appear never to attain a greater 

 diameter than 40 jm. 



