ANATOMY AND PHYSIOLOGY OF PATELLA VULGATA. 617 



side of the lamella. These cells have large nuclei, and a comparatively thick 

 cuticle. The cells are relatively far apart, the spaces between the cells being 

 filled probably with intercellular substance, possibly with sea water. The pro- 

 toplasm of the cells is clearer, but with larger granules in the superficial than 

 in the basal portion. The nucleus lies in the basal portion. Towards the free 

 extremity of the lamella the cells are more flattened and more vacuolated, 

 and the cuticle is not so distinct. All parts of the lamella and the ventral 

 face of the mantle not occupied by the muscle and connective tissue frame- 

 work above indicated are filled by a network of delicate fibres and films, the 

 cavities in which are lacunar blood spaces, and are filled with blood corpuscles, &c. 

 In the basal portion of the lamella the muscle which underlies the epithelium 

 may be seen to be continuous with the muscle of the mantle. 



At intervals along the mantle edge there are to be found papillae, probable 

 tactile in function, sunk beneath the surface. The papillae are on an average 

 about -35 mm. long, and about half that in breadth. They are conical in 

 shape (PI. CLI. fig. 31), and spring from a broadened base. They lie in pits, 

 which are sunk beneath the surface about half a mm. The pits are slightly 

 broader than the papillae, and have a narrow outlet to the exterior. In some 

 sections the papillae can be seen cut tangentially when it is seen to be completely 

 surrounded by the mantle tissue. The walls of the pit and of the papillae itself 

 are covered with epithelium, which is continuous with the epithelium lining the 

 dorsal and ventral surface of the mantle. In Patella vulgata they are about 100 

 in number, arranged apparently in a single row. 



The epithelium of the dorsal wall of the mantle, just before it bends in- 

 wards to form the pit, is very regularly columnar. The cells are widely separated 

 by cement substance (PI. CLI. fig. 32), which spreads out over the surface 

 to form a cuticle, with which the free ends of the epithelial cells are fused. 

 The nuclei are oblong. The epithelium covering the pit walls and the papilla 

 is very irregular. The cells are widely separate and irregularly columnar. The 

 surface of the papilla is much corrugated, and the epithelium is thus thrown 

 into folds which run circularly round the papilla. The epithelial cells are 

 attached by fine processes to the subjacent tissue (PI. CLI. fig. 33). The 

 centre of the papilla is composed of muscular fibres running longitudinally in 

 the papilla, and spreading in a fan-like manner, so as to unite with the 

 epithelium with their ultimate fibrillae. The muscle fibres are continuous with 

 those of the mantle. The very centre is occupied by a large nerve. There is 

 a nerve plexus, more or less distinct at the root of the papilla. 



The papilla lies nearer the ventral side of the mantle, and the inside wall of 

 the pit on that side is lined by squamous epithelium. 



(6) Renal System. — The kidneys (nephridia) are two in number, right and 

 left. Their position and relationship have been more fully worked out than 



VOL. XXXII. PART III. 5 K 



