epidermis 



blood vessels 



sculptured layer connective tissue ^ 



of dermis ^f 



■_jLi(Ai,:Tn. 



<^ ^i}^'^))^ ^k ^m:im ^ ^ V 



hyodentine 

 fibrous basal layer 



bony base — 



denticle 



^i>^ 



Figure 8-39. Structure of teleost scales. A, section of scale of teuciscus tu^\\\j%; B, joint between 

 two armor plates of a catfish Hypostoma; C, section of scale and skin of Arapaima gigas; D, surface 

 layers of a scale of the catfish, Ca/lichfhyes tongifillis. (A after Hose; B after Rauther; C and D after 

 O. Hertwig) 



scale (this is an ontogenic progression from the rhombid 

 interrelationship to the cycloid). The anterior margin is ex- 

 tended forward below the next anterior scale. Growth is 

 largely marginal, and continues throughout life. The surface 

 of the scale is decorated with small circular ridges marking 

 the growth periods. 



In Lepisosleus, the lateral-line scales ossify first in the tail, 

 then in an anterior progression (Figure 8-42). From this 

 line, scale rows extend up and forward and down and back 

 (or down and forward). The caudal peduncle is fully cov- 

 ered by scales before they appear above, or below, the 

 lateral line anteriorly. The development of the scales paral- 

 lels that of Polypterus. 



In most teleosts, the first scale papillae or buds appear 

 along the lateral line on the caudal peduncle and later ex- 

 tend forward along that line. In centrarchids, the row of scale 

 primordia extends forward below the lateral line; this 

 appears to be a deviation from the primitive style. 



Choanate fishes 



The crossopterygian fishes have a scale identified by the 

 term cosmoid (Williamson, 1849) referring to the "decora- 

 tive" nature of the bunches of dentinal tubules in these 

 scales. In this type, the surface is covered with enamel-clad 

 tubercles joined one with another to form a surface broken 

 only by pores (Figure 8-43). Below the enamel is a layer of 

 dentine, the cosmine, in which the dentinal tubules radiate 

 outward and upward from the central pulp cavity of each 

 tubercle. Below the dentine is spongy bone permeated by 

 many anastomosing channels, and below this is lamellar 

 bone. On the anterior overlapped margins of the scale, one 

 observes typical denticles, and several generations of these, 

 one over another. 



Irregular concentric rings mark the surface of these scales 

 and represent resorption and surface replacement areas in- 

 volved with growth. On the outer surface, new layers of 



238 • THE SKIN AND ITS DERIVATIVES 



