IX 



DOGFISH 



295 



and the ordinary fishes in which the caudal fin is symmetrical its upper 

 and lower halves being alike so far as external appearance is concerned. 



The surface of the body of the Dogfish, as of any other typical verte- 

 brate, is covered by a protective skin consisting of the persistent ectoderm 

 or epidermis resting upon the dermis, a layer of connective tissue 

 toughened by strong fibres running through it in all directions. It is 

 this latter layer which gives to the skin of many vertebrates, especially 

 when tanned and converted into leather, its remarkable toughness. 

 The epidermis consists of several layers of cells. The cells of the inner- 

 most layer are in a state of active growth and multiplication while those 

 of the outer layers are more or less degenerate, flattened in form and 



FIG. 122. 



Placoid scale, as seen in longitudinal section. A and B, Early stages ; C, fully-developed scale. 

 b.p, Basal plate ; d, dentine ; d.p, dermal papilla ; e, enamel ; e.o, enamel-organ; p, pulp. 



having their cytoplasm and nucleus stiffened and horny, so that they 

 form a protective layer covering in the soft active cells of the deeper 

 layers. 



The skin possesses a highly characteristic skeleton in the form of 

 placoid scales, of the greatest possible interest for in them we see, making 

 its appearance for the first time in the series of existing vertebrates, the 

 bony tissue which is destined to play such an important part in the 

 skeleton of the higher vertebrates. 



The placoid scale (Fig. 122, C) consists primarily of a hollow back- 

 wardly projecting spine composed of a special variety of bony tissue 

 named dentine (d). This is distinguished by the following characteristics : 

 (i) that its cells (odontoblasts) are arranged side by side in a single layer 

 lining the inner surface of the spine, (2) that calcification of the matrix 



