61 



prominent otolith, marked with concentric and radiating 

 lines, exactly spherical in form, and about 0'02 mm. in 

 diameter, is present. The wall of the otocyst is composed 

 of a single, rather irregular, layer of cells ; at the extremities 

 of the long diameter are two single, nucleated cells, the 

 cell bodies of which stain deeply, and on either side of 

 each of these are several clear, apparently non-nucleated, 

 spindle cells. The remainder of the wall is composed of 

 irregular cubical cells. Hairs or cilia are not clearly 

 shown. The nerves seem to enter at the internal poles 

 of the organs. Each otocyst is surrounded by a loose 

 investment of fibrous connective tissue, and but for this, 

 lies freely among the transverse muscles of the foot. 



The Eeproductive Organs. 



The gonad is paired and consists of a branching tubular 

 gland. The external opening (see fig. 3, PL I.) is situated 

 on the lateral and posterior body-wall, near the origin of 

 the posterior retractor muscle of the foot. This leads into 

 a very short terminal duct which immediately branches 

 into three main divisions (fig. 3). One of these runs 

 dorsally along the posterior margin of the viscero-pedal 

 mass, the second downwards along the body-wall, the 

 third duct runs obliquely forwards and downwards towards 

 the bend of the foot. Branches are given off laterally 

 from all these ducts on which are borne botryoidal clusters 

 of secreting alveoli (fig. 37, PL VI.). These lateral 

 branches penetrate among the transverse muscle bundles, 

 between the convolutions of the intestine and between 

 the lobes of the digestive gland. They lie principally 

 in the peripheral zone of the proximal limb of the viscero- 

 pedal mass. There is no extension at any time into the 

 distal limb of the latter, nor into the mantle lobes. 



In Cardium edule the sexes are separate. In at least 



