ANATOMY AND PHYSIOLOGY OF PATELLA YULGATA. 615 



two or more lamellae. Under the low power they appear as flattened pockets 

 more or less triangular in form, with a base attached to the mantle, and with the 

 apices projecting into the valley between the mantle and the foot (PL CLI. 

 fig. 38). 



The structure of the mantle and of the gills is so similar, and since, as shall 

 be afterwards shown, the mantle is also respiratory in function, it may be best 

 to describe them together. 



Under the low power (PI. CL. fig. 24) a vertical section shows the mantle 

 to be attached to the circular muscle just where it springs from the shell, and 

 to extend as a plate thickened in its outer third, and having certain processes 

 springing from its ventral surface, which are vertical sections of gill lamellae. 

 The entire surface of the mantle is covered by epithelium, which is in many 

 places greatly wrinkled. The mass or body of the mantle consists of connective 

 tissue and muscle, with large and small lacunar spaces. The lamellae are also 

 hollow, and their opposite walls are connected by transverse bands of tissue. 



When the epithelium is examined under a high power, it is found to be 

 columnar, but presenting variations in structure at different parts. 



The epithelium over the dorsal surface of the mantle is low columnar, with 

 large nuclei. Towards the attachment of the circular muscle it becomes 

 squamous, which again becomes continuous with the dense epithelium covering 

 the surface of the circular muscle. Outwards, the cells become longer and 

 more tapering. They are arranged in fan-shaped masses owing to the corruga- 

 tion of the surface. The tapering ends (PI. CL. fig. 25) are individually 

 attached to the fine ends of transverse muscular fibres, while the free ends, 

 which are more granular than the bodies of the cells, are covered with a 

 continuous homogeneous cuticle. Just beneath the epithelium, and separat- 

 ing the fine tapering extremities of the cells from one another, lies a layer 

 of muscle which runs in a circular manner round the mantle edge, i.e., in 

 vertical section the ends of these fibres are cut across. These fibres are 

 extremely close to the epithelium, and are not separated from it by any base- 

 ment membrane or connective tissue. Beneath this layer, and closely applied 

 to it, is another layer, which is arranged vertically, i.e., parallel with the long 

 axis of the section, in which they are seen as strands. The transverse fibres 

 run between these fibres, crossing them at right angles, to be attached to the 

 tapering ends of the epithelial cells. Beneath that layer there is a quantity 

 of connective tissue of loose texture, which is succeeded by another but much 

 thicker layer of vertical muscle fibres, which is prolonged downwards to the 

 very edge of the mantle skirt. 



At the edge of the mantle the epithelium becomes low and cubical, and is 

 frequently pigmented, the pigment being deposited chiefly as a band in the 

 centre of the cells. There are frequent indentations on the dorsal surface of 

 the edge of the mantle, the cells lining which are pigmented. 



