352 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



The scales differ much in size in different teleosts. In particular 

 cases they become greatly reduced in size (Eel) or even completely 

 disappear (many Siluroids, see p. 363). In other cases the bone forma- 

 tion in the skin becomes continuous over large areas, forming great 

 plates of bone articulated together to form a rigid coat of mail as in 

 many of the tropical Siluroids or Cat-fish. 



Of much greater importance however is the fact that in the teleostean 

 fishes the formation of bone is no longer restricted to the skin. The 



Fig. 147. 



Zones of growth in scales of Teleosts. A, Haddock at commencement of its third smnmer (after 

 Stuart Tiiomson). B, Salmon of ninth summer. 5, Widely spaced lines of growth indicating active 

 metabolism on return to sea after spawning ; s.m, spawning mark ; Wi, growth lines of first winter ; 

 Wz, growth lines of second winter. 



capacity of forming bone has spread down into the substance of the 

 body, infecting especially the layer of connective tissue adjacent to the 

 surfaces of the cartilaginous skeleton. The result is that the cartilage 

 becomes ensheathed and strengthened by a layer of the harder and more 

 rigid bone, in the form of numerous more or less flattened investment 

 bones or " membrane bones " applied close to the surface of the under- 

 lying cartilage. The process does not stop here for the bone-forming 

 connective tissue tends to eat its way into and actually replace the 

 underlying cartilage, giving rise to replacement bones or " cartilage 

 bones." 



