356 



TELEOSTEI 



the bone somewhat resembles dentine ; while in Gadus the skeleton 

 is formed of bony matrix with irregular spaces, but neither 

 Haversian canals nor bone-cells. 



The scales also almost always lose their bone-cells, and the 

 lower lamellae become transformed into thin, tough, and pliable 

 layers of crossing fibres without any vascular canals, over which is 

 deposited a more calcified layer (Fig. 193). The varied ornamenta- 

 tion on the surface of Teleostean scales is due to modifications in 

 this outer layer, in the form of ridges, spines, etc. (Figs. 342, 441, C). 

 In the ctenoid variety of scale it grows out into sharp spines situated 

 on the posterior free border (Gobius, Holocentrum, etc.) ; or over the 

 whole exposed surface (Solea, Mugil, etc.). Lens- shaped calcifica- 

 tions are sometimes scattered in the inner regions of the scale. 



Both the outer and the inner 

 layers grow by the addition of 

 new lamellae on the outside. 

 No trace of true ganoine is 

 found in modern Teleostei ; but 

 it is possible that the outer cal- 

 careous layer has been derived 

 from it. In some families the 

 scales become modified into 

 spines, either small and em- 

 bedded in the skin, or large 

 and freely projecting (Diodon- 

 tidae). Such spines do not re- 

 present modified denticles, from 

 which they differ fundamentally. 

 True denticles with dentine cone and pulp -cavity do, however, 

 occur in large numbers on the dermal skeleton of the Siluroids, 

 where they are movably articulated to the underlying bones (Fig. 

 373). This puzzling fact can at present only be accounted for on the 

 supposition that the Siluroidei have been derived from ancestral 

 Teleosts in which the dermal denticles were 1 still present, as they 

 are in Polypterus and Lepidosteus. 



The Teleostean scale develops like the ganoid (Hofer [215], 

 Klaatsch [264], Ussow [471], Ease [198]). Mesoblastic cells gather 

 together below the basement membrane, forming papillae which 

 project slightly into the epidermis. In the middle of each mass 

 of scleroblasts as it spreads out appears a thin lamella of skeletal 

 substance (Fig. 343, A). This increases in size and thickness by the 

 addition of new layers, the scleroblasts covering its entire surface. 

 The surrounding tissue becomes the loose packet in which the 

 adult scale is lodged (Fig. 193). The front edge of the scale 

 grows inwards ; the hinder edge outwards, carrying the epidermis 

 with it. 



FIG. 342. 



Ctenoid scale bf Erythrichthys nitidus, Rich. 

 (After Gtinther.) 



