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Show 144 DR. H. LYSTER JAMESON ON THE [Mar. 4, examined whole; while others again were ground down on a Water-of-Ayr hone, care being taken that the " Schliff" so made should pass as nearly as possible through the centre. The Cercarice and Sporocysts were either examined entire, or sectioned in situ. For decalcifying, it was found best to use pearls preserved in spirit, as those that had been kept in a dry state, although often giving good results when examined entire in oil of cloves, were unfit for cutting with the microtome, owing to the brittle and horny nature of the dry conchyolin. The most satisfactory effects were obtained by using very dilute (*5 to 1 per cent.) nitric acid in water. Stronger solutions often caused bubbles of carbon dioxide to be evolved in the residual conchyolin, but by using these weak solutions freely the gas was dissolved almost as soon as formed. The time required for this process varied from a few days to some weeks, according to the dimensions of the pearl and the proportion of conchyolin to salts. Structure of the Mantle and Shell. The tissues composing the mantle of Mytilus edidis are :- (1) An external simple epithelium, which is a direct continuation of the dorsal body-wall; (2) A n internal ciliated epithelium, resembling the epidermis of the foot and gills ; and (3) A spongy connective tissue, in the meshes of which the blood circulates. The external epithelium (PI. XIV. figs. 1-4, PI. XV. fig. 5, ext.ep., and text-fig. 22) is composed of a single layer of flattened columnar cells with spherical or ovate nuclei. The outer surfaces of these cells are closely applied to the inner nacreous substance of the shell. The appearance of this epidermis differs considerably according to the degree of contraction and method of preservation. Its constituent cells are polygonal in surface view, and brick-shaped or columnar in sections at right angles to the surface. Their protoplasm stains rather more strongly with haematoxylin than that of the connective tissue, and shows faint striae perpendicular to the surface. Their bases are attached to the connective-tissue fibres. Scattered here and there throughout this epithelium are spherical cells which stain lightly. They may be the " Eirund kbrnige Zellen " that Tullberg describes (1882). They are never numerous, and I a m inclined to attribute then- presence largely to defects in preservation. The epidermal lining of the mantle-cavity (PI. X I V . -fig. 2, PI. X V . fig. 5, and text-fig. 22, int.ep.) is the typical glandular ciliated epithelium so common in the skin of invertebrates. It is composed of columnar ciliated cells and interstitial gland-cells, some of which project basally into the connective tissue. It is |
