44 



STRUCTURE OF THE VERTEBRATES 



is normally homocercal, although it may assume weird propor- 

 tions in some families. The mouth is terminal and teeth are pres- 

 ent except in a few families. In the most primitive family the 

 swim bladder retains a small connection with the gut. In the 

 others the connection is lost and the organ serves only a hydro- 

 static function. 



Order Crossopterygii, which today is limited to a very few 

 genera in the Nile, Niger and Congo rivers, was once a widely 

 distributed and flourishing group. It is of particular interest to 

 the anatomist and evolutionist as it is now agreed that the 



cm 



ipiracle 



Polypterus larva with 

 External Gills 



Polypterus (Crossopterygii) 



Fig. 15. A Recent Crossopterygian Fish. The larva has the external gill 



and assumes the foot position typical of the amphibia. The order is 



characterized by the flesh}' lobe of the pectoral fin. 



amphibia arose from this order of fishes. In fact, by some anat- 

 omists, it is considered the stem form from which all the Tele- 

 ostomi evolved. The name means ''fringe fin", although "lobe 

 fin" is more descriptive of the fin condition. Instead of a fin sup- 

 ported by three parallel cartilages or bones, these animals have 

 appendages with a fleshy lobe surrounded by a fringe of dermal 

 rays. The recent forms have a rather degenerate skeleton in the 

 lobe, but their fossil ancestors had three large bones and several 

 small ones, which are clearly homologized with the humerus, 

 radius and ulna, and carpels of the land animals. 



The head plates strongly resemble those of the earliest am- 

 phibia. The primitive crossopterygians had bony plates on their 



