SCALES. 



XXI 



Fig. 5. 1, Ctenoid scale from 

 perch ; 2, Cycloid scale from lateral 

 line of gold carp ; .3, Ganoid scales 

 from amblypterus ; 4, Placoid scales 

 from small-spotted dog-fish. 



which latter may even constitute a dermal skeleton ; while it appears to be 

 commonly the case that siluroids are provided with spines for offensive 

 purposes, and fishes with electric organs are scaleless. . Scales as a rule have 

 their free edges directed backwards, to prevent their forming any impediment 

 in the water when swimming, and the different forms have been divided 

 into cycloid, ctenoid, ganoid, and placoid. 



Scales on their surface may have fine con- 

 centric striations, passing parallel with their 

 outer edge, and others more strongly marked 

 radiate from- near their centres towards their 

 attached or front border. 



Cycloid scales (fig 5, no. 2) are those desti- 

 tute of any coating of enamel, and with a 

 smooth or non-denticulated free edge. Cte- 

 noid scales (fig. 5, no. 1) differ in possessing 

 denticulations or teeth, often covering a tri- 

 angular space at their free extremity or being 

 merely restricted to the edge of the scale ; 

 these last if spiny, have been termed sparoid 

 scales, while the foregoing forms pass more 

 or less gradually one into the other; both forms may be present on 

 one species of fish. Ganoid scales (fig. 5, no. 3) are furnished with a 

 coating of enamel, mostly of a tessellated form, and often articulated 

 one to another. While placoid scales (fig. 5, no. 4) can be considered as 

 dermal growths or osseous scales, as seen in sturgeons, rays, and some 

 other forms of fish. 



Scales of fishes increase in size or grow along with the body of the fish on 

 which they are placed : thus in the salmon or trout the adult has the same 

 number as the young, while they are imbricate to the same extent. The 

 skin consists of the outer layer or epidermis, below which is the true skin, 

 termed dermis or cutis. The epidermis is originally formed by cells, which 

 increase in numbers and constitute an outer and a deeper layer. The cells 

 in the outer layer gradually flatten, and become partially cornified ; while 

 the deeper or malpighian layer is in a more active state of existence, new 

 cells are constantly being formed, which push the older ones towards the 

 surface. The cutis or true skin is well supplied with lymphatics and blood 

 vessels, and has a more active vitality than the more superficial epidermis, 

 and it gradually protrudes into slight elevations, which are the rudiments of 

 scales ; these increase in size, and passing backwards carry with them their 

 covering of epidermis. At this time calcareous deposits are laid down in 

 the dermis, while the superimposed malpighian stratum on its lower side 

 secretes a hard enamel-like substance. Thus both a portion of the epidermis 



