VERTEBRATES. 7 



skin, the skeleton, the muscular and nervous systems, the intestinal, respiratory, vas- 

 cular, and urogenital systenis. 



The Skin in Veetebeates. 



As we shall presently see, there is considerable difference in the structure of the 

 skin in the aquatic and the air-breathing vertebrates, but both groups agree in the fact 

 that it is invariably composed of two primary layers, the ' epidermis ' and the ' corium.' 

 The former is derived from the epiblast of the embryo, while the latter is formed from 

 the most superficial stratum of mesoblastic cells. Fig. 6 represents diagrammatically a 

 section through the skin of a fish, in which the two layers are very distinctly seen. 

 In the epidermis we observe that those cells which are closest to the surface are more 

 or less flattened, while those resting on the corium are much more elongated. The 

 cellular structure is always easily recognizable, the individual cells being separated by 

 chinks which are traversed by connecting bands of protoplasm. Occasionally a 

 branched pigment-cell may be seen thrusting its processes into these inter-cellular 

 chinks. Certain of the surface cells are constantly being transformed into mucus- 

 cells, which, becoming larger than their neighbors, and undergoing a chemical and 

 physical change, eventually burst and discharge their mucous contents on the sur- 

 face of the skin. In those fishes, like the eel and cat-fish, where the skin is particularly 

 slimy, this is no doubt due to the abundance of certain very large ' clavate * cells, which 

 pour out their seci-etion into the intercellular spaces. Such clavate cells are known in 

 the common mud-puppy and other aquatic Amphibia, but they disappear from the skin 

 with the change to a new element in cases like the salamander, which, after a larval 

 aquatic life, become, when adult, wholly terrestrial in their habits. As for the mucus- 

 cells, they also do not occur in the skin of air-breathing animals, although they are 

 abundant in the cavity of the mouth. 



The epidermis and corium are not in contact with each other by a perfectly flat 

 sui'face, but the latter is here and there elevated into ' papillae,' in which the looped 

 capillary vessels are found, whence the nourishment of the epidermis is obtained, and 

 through which the nerves which are destined for the epidermis course. Below this 

 papillary layer, which is further generally remarkable as a seat of pigment cells, the 

 corium is formed, in the aquatic vertebrates, of pai-allel bands of fibres, traversed here 

 and there by a stout bundle conveying outwards vessels and nerves. Separating this 

 comparatively dense layer from the underlying muscles, is a varying amount of looser 

 connective tissue, often rich in fat, which is termed the ' subcutaneous connective 

 tissue.' 



In many respects the epidermis of aquatic vertebrates is a higher and more com- 

 plex organ than that of the air-breathing forms. This is especially true of modified 

 tracts of the epidermis, which are developed at the extremities of nerves, and which 

 are on this account known as ' neuro-epithelial ' structures. Two varieties of these 

 exist in the skin ; one, on account of its shape and its relation to a nerve, is known as 

 an ' end-bud,' and such end-buds, carried upon papillae, are extremely common over the 

 whole of the skin, but especially on the head and in the cavity of the mouth. Although 

 such end-buds are not found in the skin of air-breathing vertebrates, yet they persist 

 in the mouth, where, from being organs of a more general tactile function, they are 

 specialized as organs of taste. The second category embraces a very important group 

 of sense organs, the nerve-hillocks or neuromasts, which are found in all fishes and 

 aquatic amphibians ; they differ in shape, as well as in the form of the sense-cells, from 



