On the Visual Orgaus in Lamellibranchiata. 453 



serve to retract the edge of the mantle withiu the shell , so that it will 

 not be injured when the two shells meet. 



Besides these retractor muscles are seen the extensor muscles 

 (fig. 1 e.m.) , which in the section are cut trausversely. This muscle 

 when it eontracts forces the mantle outward. These muscles naturally 

 are placed on the internal surface (fig. 1 B.S.) of the mantle and are 

 attached at intervals to the shell. 



Covering the whole externa! surface of the mantle is an epithelium 

 of columnar cells which may be classed under two heads, namely : that 

 which lies in contact with the shell and cuticle (fig. 1 s.e.), and that 

 lining the mantle which is directed toward the branchial cavity (fig. 1 

 o.e.). The former {s.e.) are much longer and larger, and the surface on 

 which they are situated is thrown into a great number of folds , these 

 are probably stretched out when the mantle is fully extended. These 

 cells are very slender and it is very difficult to distinguish the separate 

 cells. The nuclei, which have the shape of long spbaeroids , are 

 situated at the deeper third of the cell and are very small and very 

 narrow. These cells scerete the nacre which forms the internal layer of 

 the shell. 



The branchial epithelium (b.e.) , which for us is the most impor- 

 tant, forms a strong contrast with that lying next to the shell. The 

 cells forming it are much smaller but more distinct. This epithelium is 

 not thrown into folds as the other , but forms a continuous layer on the 

 internal wall of the mantle. 



These so-called palliai epithelial cells , however . are not all alike, 

 for if we examine them closely we find a number of them pigmented. 

 The cells containing pigment seem to be most numerous and best de- 

 veloped betweeu the inner two folds (fig. 1 b and c] of the mantle bor- 

 der. Pigmented cells are also found scattered along the palliai side of 

 the mantle, and these are the cells which form the black band before 

 mentioned. 



In examining these pigmented cells with a high power we find 

 them very peculiarly constructed. They are columnar cells possessing 

 a very large and almost spherical nucleus. At the free end, or that part 

 directed toward the exterior world, we find that there is a collection of 

 fine pigment granules of a dark brown color (fig. 2) . This pigment 

 occupies the outer fourth of the celi and does not seem to be of equal 

 thickness , being thinner in the centre or axis of the celi. The rest of 

 the celi is fiUed with a granulated protoplasm in which is embedded 

 the large nucleus. The nucleus is nearest the tunica (fig. 2 t) on 



