THE AMPHIBIA 89 



The parting of the ways that separated the first terres- 

 trial animals, the Devonian Amphibia, from the air- 

 breathing fishes depended not so much on the e£Bciency 

 of aerial respiration as on the structure of the fin that 

 was to become a foot. 



A fish propels itself primarily by means of movements 

 of its tail and body, the fins serving chiefly to maintain 

 an even keel, to prevent pitching, and to improve the 

 accuracy and speed of turns. In the Devonian fishes the 

 pectoral and pelvic spines had become connected with 

 the underlying muscles and articulated with the pectoral 

 and pelvic girdles, to supply one pair each of pectoral 

 and pelvic fins as these appear in the higher fishes. Pro- 

 fessor Romer has remarked that there is nothing sacro- 

 sanct about four as a limb count, and it is amusing to 

 speculate about the possible results had some of the 

 spiny sharks, with up to seven pairs of ventral append- 

 ages, survived in the higher animals. (Again, science 

 fiction writers may take notel ) 



Among the Devonian bony fishes the fins had lost the 

 spines of the placoderms and acanthodians and devel- 

 oped a basic pattern, which had started to evolve in two 

 directions. In one group the fin developed many fanlike 

 rays anchored to a long base, thus giving rise to the 'ray- 

 finned' fishes (Actinopterygii, aktinos = lay; pteron = 

 wing; pterygion = little wing or fin) as represented by 

 the common fishes of today. This many-rayed fin was 

 excellent for swimming— but it was too feeble to support 

 the weight of the animal out of water, and very few of 

 the Actinopterygii could crawl even short distances on 

 land. In the few ray-finned fishes that essay to scramble 

 out of water today— such as the mudskipper, Periophthal- 

 mus, and the Indian climbing perch, Anabas—^e fins 

 have secondarily been remodeled into relatively feeble 

 crawling or climbing appendages. 



In the other group, however, the primitive fin de- 

 veloped into a more compact paddle- or tassel-like struc- 

 ture, giving rise to the Crossopterygii {krossoi = tassels; 



