CHAPTER XXII 



FINS AND LIMBS 



The most primitive chordates relied for their locomotion on 

 the myotomes of the body-wall, which, by contraction on one 

 side and relaxation on the opposite side of the body, can produce 

 the sinuous bendings which pass like waves down the length of 

 the body and propel the organism along. Amphioxus is in 

 this condition. 



Improvement of methods of locomotion is connected with 

 the formation of extensions of the body in the shape of fins. 

 The earliest of these to arise were apparently those which lie 

 in the middle line of the dorsal and ventral surfaces : the 

 so-called median fins. In Amphioxus they are foreshadowed, 

 but in Petromyzon well-developed median fins are present, 

 supported by cartilaginous radials provided with radial muscles 

 at their base on each side. Fish likewise have median fins, 

 and these show an advance over the conditions in Petromyzon 

 in that the web of the fin is supported by dermal fin-rays in 

 addition to the cartilaginous radials. These fin-rays are horny 

 (ceratotrichia) in the Selachians ; bony and jointed (lepido- 

 trichia) in the Teleostomes, and in the Dipnoi they are fibrous 

 and jointed (camptotrichia). The median fins of the amphibia 

 have neither cartilaginous radials nor dermal fin-rays at all, 

 and in some of them the fins develop and regress according 

 to the season and the breeding period. 



In the fish, in addition to the median fins there appear the 

 two pairs of " paired " fins : a pectoral pair and a pelvic pair. 



The method of origin of median fins and paired fins is very 

 similar. In each case a longitudinal fold of skin appears, 



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