STRUCTURE OF THE VERTEBRATES 45 



bodies, although the recent forms have overlapping scales. The 

 spiracle is retained. The swim bladder is a ventral outpocketing 

 of the gut, and may be either one or two lobed. The bladder is 

 highly vascular, the blood supply coming from the last (sixth) 

 gill artery in much the same way that the lungs of the higher 

 forms is supplied. 



Aside from the fossil and anatomical evidence, the larva of 

 the recent forms shows relationship with the amphibia. Unlike 

 most fish, the crossopterygian hatches as a larval form with 

 external gills in addition to the normal internal. In this they 

 resemble the amphibian tadpole. Equally as interesting is the 

 fact that the larva settles to the muddy bottom with its front 

 fins drawn under the body, using them as a support, not merely 

 as a balancing organ as does the typical fish. After metamorphos- 

 ing from the larva to the adult type, the fish makes constant use 

 of its respiratory swim bladder, for its normal habitat is in the 

 muddy rivers and lakes which tend to become de-oxygenated 

 and unhealthy for gill breathing animals. Due to its importance 

 in the scheme of human evolution, frequent reference will be 

 made to this order in the later chapters. 



Dipnoi. This small group has been given equal rank with the 

 two great sub-classes, although the living representatives are 

 limited to three genera. This is on account of their wide diver- 

 gence anatomically, and their ancient lineage as a widely dis- 

 tributed, definite, easily defined division of vertebrates. All re- 

 cent forms live in tropical regions which have seasonal periods 

 of rain and drought, one in Africa, one in Australia, and one in 

 South America. Most anatomists agree that they are a group 

 which has paralleled the amphibia in evolution; a trial, and it 

 appears now largely an error, on the part of nature in evolving 

 land animals. 



From earliest times these animals were highly specialized as to 

 teeth and fins, two characters which effectively prevented further 

 evolution toward land life. The eggs hatch as larvae with ex- 

 ternal gills. During larval life the lungs develop from a ventral, 

 paired, highly vascular swim bladder. These lungs are more 

 lobulated, larger, and more efficient than those of most amphibia. 

 This is an adaptation to the dry seasons during which the 



