12 



VERTEBRATE SKELETON 



the young which are lost in the adult (except at the base of the 

 tail), unless scattered irregular calcareous bodies are degenerate 

 scales. The scales of Amia lack all ganoin and, in arrangement 

 and appearance, are very similar to the cycloid scales of Teleosts 

 (infra). At the bases of the fins of Ganoids (and a few Teleosts) 

 are modified scales known as fulcra. 



TELEOSTS. — ^Many but not all eels and scattered members of 

 other groups lack scales, but most of the others have them arranged 

 in rows in line with the myotomes (occasionally two or more rows to 

 a myotome), the successive rows overlapping like shingles, from in 

 front backwards (fig. 9). In their development mesenchyme cells 

 become aggregated at regular intervals, the interior cells of each 

 group becoming specialized as scleroblasts which secrete a thin 



Fig. 9. — Relations of teleost scales (black) 

 to layers, c, corium; e, epidermis. 



Fig. 10. — Cycloid and ctenoid scales. 



layer of organic matter from their inner ends. This secretion 

 increases with age by the deposition of successive layers, each new 

 layer extending beyond the margin of the last, the result being a 

 series of lines of growth. At first these scales are remote from each 

 other, but with growth, the anterior margin of each extends shghtly 

 inwards, the posterior outwards, resulting in the characteristic 

 overlapping (fig. 9). Usually, but not always, the scales contain 

 bone cells. The inner layers are thin and flexible, the outer more or 

 less calcified. 



Most Teleost scales (fig. 10) may be grouped as either cycloid 

 or ctenoid, but the two types intergrade and both may occur on the 

 same fish. Cycloid scales, occurring in Physostomi and many 

 Acanthini, are approximately circular and are marked by radiating 

 lines and concentric fines of growth, the centre being a fittle behind 

 the middle of the scale. Ctenoid scales have the same fines and also 

 radiating series of spines on the outer surface, these covering the 

 whole side or only the posterior portion. Some Siluroids have 

 denticles (with pulp cavities) in the skin, recalfing the larval Lepidos- 



