79 



Dr. Eamsay H. Traqnair, published in 1865.*" The sole is not considered in this 

 paper, but the interpretation of the distribution of the organs and canals in this 

 species is comparatively easy after Dr. Traquair's lucid explanation of the derivation 

 of the arrangements found in other flat-fishes from the original symmetrical condition. 

 The diagram of the arrangement in the cod which I have given is copied, with 

 slight modifications, from Dr. Traquair's ; and my diagram of the arrangement in the 

 sole is constructed on the same plan as his diagrams of the arrangements in the plaice 

 and other species. 



Minute Structure of the Skin, Dermal Tubes, and Sense-organs. 



Wlien thin sections of the skin are prepared and examined under the microscope, 

 the derma is seen to be composed of a number of sheets and bands of felted fibres, as in 

 PI. XIV, 6. In my preparations no nuclei are visible in this tissue, but the pieces of 

 skin were decalcified in weak nitric acid before they were cut, in order to remove the 

 lime from the scales, and the action of the acid may have somewhat altered the 

 condition of the fibrils. The scales, as seen in section, consist of thin laminae lying one 

 upon another ; the laminae are entirely homogeneous in structure, and seem to be 

 simply sheets of the fibrous tissue which have been consolidated and then impregnated 

 with calcareous salts. The scales are contained in cavities of the fibrous tissue. In the 

 preparations the fibrous tissue is separated slightly from the surface of the scales, but 

 this is doubtless due to the shrinking produced by the process of preparation, and in 

 life the fibrous tissue is in contact with the surface of the scale. Above the scales 

 there is a layer of delicate spongy fibrous tissue in which the fibres are short, and run 

 in a vertical as well as a longitudinal direction; this reticular tissue contains 

 numerous nuclei. This tissue separates the external surface of the scale from the 

 dense fibrous laminated tissue, while the internal surface of the scale is in immediate 

 contact with the latter. The epidermis consists of several, six or seven, layers of cells. 

 At the base of the epidermis the cells are polygonal and as broad as they are high : 

 each cell contains a large nucleus, which becomes deeply stained under the action of 

 staining liquids. Towards the outer surface most of the cells become thin and flat, 

 as in the human epidermis, but in preparations a certain number of them are large 

 spherical vesicles. These are mucous cells. The surface of the sole as of most other 

 fishes is. during life, always covered with a certain amount of slimy mucus which ia 

 derived from the epidermis. This mucus is produced by the discharge of the contents 

 of the globular mucous cells just mentioned. The cells at the base of Ihe epidermis 

 are constantly multiplying and growing, and in consequence the outermost of them 

 are gradually pushed to the surface. The superficial layers are as constantly broken 

 down and, as it were, dissolved away, some of the cells becoming, before they reach 

 the surface, converted into capsules of mucus, and this mucus, together with the 



* "On the Asymmetry of the Pleuronectidae, as elucidated by an Examination of the Skeleton in 

 the Tarbot. Halibat, and Plaice." (" Trans." Lin. Soc., vol. xxv.) 



