550 ANATOMY OF VERTEBRATES. 



The varied and often brilliant colours of fishes are due to 

 pigment-cells at different depths of the skin, but chiefly in the 

 active or differentiating area : those of silvery and golden lustre 

 are mostly on the surface of the scales : the silvery pigment, called 

 s argentine,' is an article of commerce, used for the colouring of 

 factitious pearls, and offers a crystalline character under the 

 microscope. The blue, red, green, or other bright-coloured pig- 

 ment is usually associated with fine oil, and occupies areolie 

 favouring accumulation at, or retreat from, the superficies, and 

 thus effecting changes in the colours of the fish, harmonising 

 their exterior with the hue of the bottom of their haunt. 1 



The surface of the body is lubricated in most Fishes by 

 mucus, sometimes, as in the Eel and Burbot, forming a thick 

 layer. The skin of the Eel is perforated by numerous ducts 

 or follicles, which contribute to this excretion. 2 In the Pike the 

 scattered ducts notch the border of the scales, near their termina- 

 tion : one series of follicles represents the lateral line, as in the 

 Eel. In most Fishes the follicles of the lateral row are connected 

 by a longitudinal canal, of which they appear to be branches ; more 

 particularly so in those species (Dory, Opah) in which the follicles 

 are produced into secondary tubes, and open at some distance 

 from, usually beneath, the lateral canal. In the Mugil cephalus 

 there are several lateral canals, giving off the follicles which 

 tunnel the scales in their outward course. The lateral canal 

 itself so marks the scales along which it runs, its follicular outlets 

 perforating them. The ' nervus lateralis ' sends a filament to 

 each scale-follicle or tunnel ; 3 the cephalic system of well-nerved 

 mucous canals excavates oddly superficial bones of the head in 

 many Teleostomi:* this system is noticed \n. Plagiostomi, at p. 225. 

 The nervous structure connected with the system of the lateral 

 line suo-ffests a stimulus to active excretion from emotional causes, 



o~ * 



as in the skin-glands of Batrachia. 



100. Teguments of Reptiles. The derm in Batrachia presents 



me to the meeting of the British Association in August 1838, and to the Academy of 

 Sciences, Institute of France, December 1839, established the 'conversion-theory,' and 

 compel any one attempting to revive the ' excretion theory ' to substitute a definition of 

 the process wholly different from that to which De Blainville and other supporters of the 

 theory held in 1838-9. Prof. Williamson, who has pursued microscopical investigations 

 into the scales of recent and fossil fishes with perseverance and success, affirms the facts 

 which he brought forward (ccxci. p. 437) 'to be, at least, sufficiently conclusive to 

 settle the question, by showing that, whilst the scales are formed, as originally stated 

 by M. Agassiz, by the apposition of successive layers, these layers are not generated 

 by any process of secretion, but by the calcification of an organised basis, resembling 

 that of bones and teeth, as asserted by Prof. Owen.' 



1 ccxciv. p. 327. 2 ccxcu. 3 Lcydig (CCLXXVII. p. 203, fig. 108) describes 

 it as ending in a kind of ganglion. 4 ccciv. p. 171. 



