MEDIAN FINS 31 



ures, i.e. as ridges in the direction of the fish's axis or 

 line of motion. 



Fish fins have long been distinguished as vertical (me- 

 dian, or unpaired) or lateral (paired), the former function- 

 ing both as keel and means of propulsion, the latter as 

 accessory and specialized balancing organs.^ 



Median Fins 



Median fins are unquestionably the older. They exist 

 in the simplest condition in those fishes whose axis is long 

 and whose motion is undulating. Indeed, the sole swim- 

 ming requisite is here the continuous dermal keel which 

 passes down the back from the head to the body terminal, 

 and extends thence forward on the ventral side. The 

 undulatory motion of the body is well transmitted to the 

 surrounding medium by the exaggerated undulation of 

 this long, waving fin web. This condition was probably 

 the ancestral one in the evolution of fishes. It represents 

 the simplest metamerism ; it occurs as the adult condition 

 in the lampreys (p. 57), and as the embryonic or larval 

 stage in all fishes, appearing before any traces of paired 

 fins are known ; .it is even adverse to their specialization : 

 should life habits require undulatory motion, paired fins 

 must inevitably tend to disappear (eel, p. 173 ; Cala- 

 moichthys, p. 150). 



From this condition the further evolution of the un- 

 paired fins may thus be theoretically outlined. 



The primitive continuous dermal fin could have been 

 of little value in active movement : its more rapid undu- 

 lations could not have greatly increased the rate of motion, 

 since its web, lacking in supports, would not have retained 

 its rigidity. As the simplest means of strengthening the 

 fin fold, " actinotrichia " (Ryder), appear to have been early 



