CAUDAL FIN 



35 



like rays, which prove stronger and more serviceable than 

 the heavier radials ; it seems more capable of adapting 

 the fin for special uses. 



Accordingly, in many forms of recent fishes, notably 

 bony fishes, the entire fin is found to become of dermal 

 origin ; the radio-basals, greatly reduced in number and 

 size, extend no further outward than the base of the fin ; 

 they are usually small and irregular, and are often deeply 

 sunken within the body wall. 



After this glimpse at the mode of origin of the vertical 

 fins, i.e. dorsals and anals, the history of the final vertical 

 fin, the tail, and of the paired fins may next be reviewed. 



TJie Caitdal Fin 



The tail, or caudal fin, is the main organ of aquatic 

 propulsion, and it is doubtless on this account that it 

 presents so wide a range in its structure and outward 

 form. From the earliest times there are found fishes of 

 all groups whose tail shapes are tapering (diphyceiral, Fig. 

 47), unsymmetrical {heterocercal, Figs. 45, 46), or squarely 

 truncate {Jwmocercal, Fig. 48), as the mechanical needs 

 in swimming may have demanded. 



The following summary of the mode of evolution of 

 the caudal fin seems to be warranted by study of fossil 

 and embryonic forms. The vertical fin fold of the ances- 

 tral fish was probably carried around the body terminal 

 and strengthened by constant actinotrichia (Fig. 39 C), a 

 condition similar to that (Fig. 44) of an early larval 

 stage of living fishes {protocercy). This caudal structure, 

 however, could have proven of value only in sluggish 

 undulatory motion. The functional needs, which gave 

 rise to radials anteriorly, have in the tail region produced 

 firmer and stouter fin supports. These appear both on the 



