32 



EVOLUTION OF FINS 



evolved (Fig. 39, T) ; these are slender, unjointed fin sup- 

 ports, passing from the body wall to the margin of the 



fin, appearing to arise without relation 

 to the underlying body segments. The 

 more rapid undulations of the contin- 

 uous fin would next cause nodes to 

 arise ; and at other points the greatest 

 mechanical stress would occur. Tlvese 

 portions of the fin web would accord- 

 ingly become prominent, while the in- 

 tervening or useless parts would dimin- 

 ish in width and tend to disappear. The 

 body terminal (tail, caudal fin) has now 

 *• become the seat of propulsion : dorsal 

 and ventral fins arise as lobate elements 

 of the fin fold, functioning as vertical 

 keels in the region of the body where 

 mechanical stress demands them (v. Fig. 

 40), increasing in size as the intervening 

 portions of the web gradually disappear. 

 Their rate of growth is doubtless af- 

 fected by the appearance of the paired 

 fins ; for even at an early period of de- 

 velopment these are known to have an 

 important function in balancing the fish. 

 The lappet-shaped fins (Fig. 40) next 

 acquire more rigid supports. Cartilagi- 

 nous rod-like elements arise within the 

 fin web, arranged in metameral sequence, 

 representing, perhaps, fusions of actino- 

 trichia. As shown in Fig. 40, these car- 

 Fig. 39- — Hypothet- tilaginous '' 7'adials,'" R, appear to be 



ical ancestral shark. Let- , ^ • ^^ • ^ ^ 



ters as on p. 33. largest and stoutest m the widest por- 



:•••(? 



■:■.•: 



