Polypterus The scales of Polyplerus are of the primitive 

 ganoid type, without Williamson's canals (Figure 8-36). 

 There is a thin but stratified layer of enamel over the outer 

 surface, a layer of dentine above a vascular plexus enclosed 

 in spongy bone, and a laminated bony base; the scale is 

 penetrated from top to bottom by a few vascular channels. 

 These scales have many canals for the Sharpey's fibers 

 binding them together. Small spines or denticles may over- 

 lie the margins of the scales but are not attached to them. 

 The dermal bones and fin spines differ only in that areas of 

 dentine are intermixed with the enamel. The denticles may 

 fuse to form separate (from the bony rays) plates on the fins 

 or the denticles may fuse with and become embedded in the 

 bony segments of the rays. 



Cbondrostean Acipenser and Polyodon have scales shaped 

 like those of primitive ganoids (rhomboid). Acipenser has 

 large scutes along the dorsal midline and along the lateral- 

 line canal. The scales and plates have a glassy surface layer 

 of acellular bone (hyodentine) raised into small tubercles, 

 and irregular ridges. 



Holostean Lepisosteiis has thick heavy scales of a rhomboid 

 shape. The Williamson's canals, or the lepidosteid tubules, 

 are reduced to channels for protoplasmic processes (Figure 

 8-37). Cells still remain, however, in the basal bony laminae. 

 The dentinal layer is lacking and several layers of enamel 

 cover the outer surface. At the posterior and ventral margins 

 of the scale, secondary enamel-clad denticles are formed 

 which attach to the posterior and ventral bony margins of 



the scale and become embedded in the enamel as the scale 

 grows peripherally. 



The scales of Amia are greatly modified (Figure 8-38). 

 The outer surface is ornamented and formed of a bony 

 material containing cells only in its anterior (overlapped) 

 part. Below this calcified layer is a series of fibrous laminae 

 with embedded cells. These fibrous laminae correspond to 

 the laminar bone of other scales. 



The histological dissimilarity of these holostean scales can 

 be inferred from their difference in shape. Those oi Lepisosteiis 

 overlap only slightly and employ a peg and socket in the 

 imbrication of a vertical series. The dorsoanterior peg, which 

 lies above and behind the extended corner of the scale, and 

 the ventromedial socket are characteristic of the ganoid 

 scale. 



The cycloid scale oi Anna has lost the peg and socket and 

 the Sharpey's fiber bindings. Expansion of the scale margins 

 has also been accompanied by histological changes such as 

 the loss of the enamel and dentine layers, and reduction in 

 thickness and calcification of the laminar base. An acellular 

 bone has been substituted for the enamel and dentine. This 

 transition from rhomboid to cycloid scales, along with cor- 

 responding histological changes, is observed also in the 

 choanate fishes. 



The dermal bones oi Amia (Figure 8-38) resemble those 

 of Lepisosteiis in that they have Williamson's canals penetrat- 

 ing both outer and inner surfaces. These canals allow pene- 

 tration by cell processes into the depth of the laminate 

 bone — which also contains osteoblasts. These canals indicate 

 growth occurring at both outer and inner surfaces and 



anterior 



shiny surface with growth lines 

 peg 



^peg in socket 



denticles 



/ \ 



TT-T 



sharpey's fibers 



Figure 8-36. Rhomboid ganoid scales of Polyplerus. A, outer surface of several orticulated scales; 

 B, inner surface of several articulated scales; C, longitudinal section of joint between two scales; D, 

 vertical section of joint between two scales. (After Kerr, 1952) 



236 • THE SKIN AND ITS DERIVATIVES 



