32 
EVOLUTION OF FINS 
evolved (Fig. 39, 7); 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. These 
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 “vadials,”’ R, appear to be 
ical ancestral shark, Let 
ters as on p. 33. 
largest and stoutest in the widest por- 
