212 



OSTEICHTHYES 



considerably both in their mode of growth and histological structure 

 (p. 214). In the more specialised fish they undergo modification 

 in diverse ways becoming spine -like (Gymnodontes), or few, 

 large, and plate-like (Acipenser, Loricariidae), or sometimes dis- 

 appearing altogether. 



Besides these body -scales are found scale -like exoskeletal 

 elements set end to end in rows, and forming jointed dermal 

 fin-rays, called lepidotricUa (Goodrich [175]), supporting the web 



of both the paired and the 

 median fins (Figs. 184, 185, 

 186, 187). The minute struc- 

 ture of these fin-rays is almost 

 or quite identical with that of 

 the scales of the fish to which 

 they belong. This is true more 

 especially of the lower forms. 

 In some, such as AmblypteruSj 

 there is a perfect gradation in 

 form and arrangement between 

 the body-scales and the fin-ray 

 elements. But, as a rule, the 

 transition is more abrupt, the 

 segments of the rays acquiring 

 a squarish or oblong shape, and 

 not overlapping (Fig. 249). 

 Both the scales and the seg- 

 ments of the lepidotrichia are 

 embedded in the dense con- 

 s nective tissue, the fibres of 

 which enter the substance of 

 the bone. Movable joints are 

 formed by the fibrous matrix 

 remaining unossified between 



Fia. 185. 



them (see pp. 214 and 356). 

 Large superficial dermal 



Diagram of a section through the dorsal fin of , , j i i A i 



Acipmser. a, actinotrich ; I, lepidotrich ; r, plates are developed in the 



ji (From( 2" ftrf - Osteichthyes on the head and 



shoulder - girdle, which also 



resemble the scales in structure and development. There can be 

 no doubt that these bones are of the same nature as the scales 

 on the body, though probably they were never exactly like them in 

 shape or disposition. In the course of the differentiation of the 

 now widely divergent groups included in the Osteichthyes the 

 dermal bones have undergone many important changes, yet a 

 fundamental plan can be made out which is common to all the 

 more primitive forms (Figs. 236, 240, 250-51). 



