30 



EVOLUTION OF FINS 



which apparently was imbedded in the integument ; its 

 gouge-like form (Figs. ^J and 38) permitted it to be firmly 

 apposed to its anterior and posterior neighbours. Each 

 median enamelled cusp represents apparently the sum of 

 the shagreen papillae, occurring in the median-dorsal region 

 of each metamere, its gouge-like underlying portion the 

 metameral calcification of the bases of the denticles. 



What has been the mode of origin of the primitive 

 derm cusps is a puzzling question. It is significant, per- 

 ^ haps, that they occur in primitive forms (sharks) in con- 



' nection with the sense organs of the lateral line (p. 50), 



and that they are in this region retained in a number 

 of archaic forms (Polypterus, p. 148, Callichthys, p. 172), 

 which have in all other body parts evolved protective derm 

 plates.* It is certain that for the sensory groove of the 

 lateral line, no more simple, protective devices could have 

 arisen than conical elevations of skin. Arising in this 

 region, they may have extended their protective functions 

 over the entire body surface. 



3. THE EVOLUTION OF FINS 



Fins are the organs of progression adapted to the 

 needs of aquatic living. A fish, balanced in its living 

 medium, acquires, as has been seen, a boat-like form, 

 enabling it to pierce the water in the least resisting 

 manner. Its appendages, when they come to arise, must 

 reasonably be looked to to fulfil the mechanical condi- 

 tions of aquatic motion in order to propel to the best 

 advantage the lightly balanced and boat-shaped mass. 

 Fins might thus be expected to arise as keel-like struct- 



* In the sensory canals of the head of Chimsera, the presence of scattered 

 bony plates, protective in function, v. p. 114, would suggest the concentration 

 of the marginal cusp elements for more perfect protection. 



