144 DR. H. LTSTER 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 

 shovild 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 bvibbles 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 edulis are : — 



(1) An external simple epithelium, which is a direct continua- 



tion of the dorsal body- wall ; 



(2) An internal ciliated epithelium, resembling the epidermis 



of the foot and gills ; and 



(3) A spongy connective tissue; in the meshes of which the 



blood cu'culates. 



The external epithelium (PI. XIY. 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 hfematoxylin 

 than that of the connective tissue, and shows faint strise perpen- 

 dicular 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 

 nvimerous, and I am inclined to attribute their presence largely to 

 defects in preservation. 



The epidermal lining of the mantle-cavity (PI. XIY. fig. 2, 

 PI. XY. 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 



