208 INTRODUCTION TO THE FISHES 



lines, or both combined. In these scales there is usually a 

 nucleus, which may be irregular. The radiating lines diverge 

 from the circumference of this nucleus. The parts of the margin 

 on the scale between the radiating lines usually project in little 

 convexities, and when the irregularities are limited to one end of 

 the scale, that end is usually implanted in the skin. In many fishes 

 the end of the scale is bordered with tooth-like processes. The 

 parts of a scale are defined in relation to the nucleus or focus 

 from which growth originates ; the longitudinal lines which run 

 out from this nucleus sometimes forming furrows, and some- 

 times perfectly closed tubes. The concentric lines originate in 

 the development of new cells, which become filled with horny 

 matter and ultimately arrange themselves in concentric lines. 

 It must not be supposed that the scales are on the surface of the 

 skin. The skin of a fish, like that of other vertebrates, consists 

 of two layers : an outer, cellular layer, without blood-vessels, 

 which is called the epidermis, and a thicker internal layer, fibrous 

 and supplied with blood-vessels, which is called the derma. The 

 bases of the scales are situated in this inner tissue, the derma, 

 but their posterior edges may project more or less through the 

 epidermis. 



The skin of the dog-fish will offer the student possessing a 

 microscope, and a razor wherewith to cut sections, an interesting 

 introduction to the dermal tissues of fishes. The entire surface 

 is covered with minute placoid scales, or dermal denticles (teeth), 

 as they are called, each consisting of a flat base possessing the 

 structure of bone and a pointed projecting spine. The external 

 layer of the spine consists of enamel, below which is a layer of 

 dentine or ivory, the characteristic substance of all teeth, and 

 within this layer is a cavity containing vascular pulp and open- 

 ing by an aperture in the basal plate ; in fact, the whole scale has 

 the same essential structure as a tooth. 



The late Professor Agassiz, impressed with the differences of 

 form in the scales, at one time believed that fishes might be classi- 

 fied by means of them, and he proposed to divide the scales into 

 four types : those which were bony, and formed of a thick, osse- 

 ous layer, covered with hard, transparent enamel, were termed 

 Ganoids; those dermal spines or tubercles seen in the thorn- 

 back ray and many other cartilaginous fishes, which have a spine 



