258 GILBERT C. BOURNE. 



the auimal^ its continuity being broken only by the surfaces 

 of attaclinient of the adductor muscle, the retractor muscles 

 of the foot and byssus, and the special slip of pallial muscle 

 referred to above. The edges of both mantle lobes are 

 thickened and bear small pallial tentacles covered by a 

 columnar epithelium. There are also a few much larger 

 tentacles on the posterior lower margin of the right mantle 

 lobe. There are no marginal pallial eyes or pigment spots, 

 but at the bottom of the groove formed in the thickened 

 pallial margin a track of columnar cells, supplied by twigs of 

 the circumpallial nerve, can be distinguished. In the region 

 of the visceral mass, that is to say in its upper half, the left 

 pallial lobe is very thin; but in its lower and posterior half, 

 where it covers the large recurved gills, it is greatly thick- 

 ened by the abundant development of lacunar tissue. The 

 slip of pallial muscle, attached to the left valve of the shell, 

 is shown at 'pl.ni, in fig. 2. It coincides in position with the 

 attachment of the axis of the left gill to the mantle lobe, and 

 must function as a retractor of the gill. The left gonad 

 extends backward to the level of the anus in the left pallial 

 lobe, and in the same lobe there is a large anterior extension 

 of the left gonad, forming a conspicuous sausage-shaped 

 swelling in front of the mouth (figs. 1 and 2, go. a.). 



The most remarkable feature in the left pallial lobe is the 

 presence of a number of deeply pigmented spots, arranged in 

 an irregular oval at a considerable distance from the pallial 

 margin. These eye-spots, as I must call them, vary in number 

 in different individuals. In the specimen shown in tig. 2 

 there are twenty-three. They are, however, very constant in 

 position : the most anterior eye-spot is always situated just 

 behind the surface of attachment of the anterior retractor 

 pedis muscle, and the most posterior close to the attachment 

 of the pallial muscle. Each eye-spot consists of a ring of 

 black pigment surrounding a central opaque white area. The 

 histology of these organs will be described in the latter part 

 of this paper ; but I may say here that their minute structure 

 leaves little doubt that they are sensory in function and 



