358 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



True fishes belong to three very distinct types whose common 

 ancestors have never been found. One type — the elasmobranchs — 

 is represented by the sharks and skates, with their skeleton of gristle 

 instead of bone, five or more open gill slits, and skin of shagreen. 

 The second type — the Actinopterygii — includes the teleosts which 

 form the great majority of living fishes, with a bony skeleton, the 

 fanlike fins, gill slits hidden by the large gill cover or operculum, and 

 the body covered with usually small, thin scales. The third type — 

 the Crossopterygii — comprises the coelacanths and their relatives; 

 these are thick-scaled and lobe-finned, with the skeleton part gristle, 

 part bone. 



When we found the first traces of the Crossopterygii, in Devonian 

 strata some 300 million years ago, they were already divided into three 

 different groups. It is the very different subsequent fate of each 

 of those three groups that is the crux of the coelacanth story. They 

 all had rather long, heavy bodies with thick, bony scales and lobed fins 

 common to their kind, and to that extent were somewhat alike. How- 

 ever, two of these groups had other very important characters: in 

 addition to gills, which all fishes possess, they had lungs by which 

 they could breathe air directly, and internal openings to their nostrils 

 so that they could breathe air regularly while keeping their mouths 

 shut and exposing no more than the tip of their snouts above water. 



The first of the two lung-breathing groups was the Rhipidistia, for 

 which there is no popular name as all of its fishy representatives 

 disappeared long ago. However, among the Rhipidistia was a small 

 progressive element which used their limblike fins and their ability 

 to breathe air to scramble ashore when the pools in which they lived 

 started to dry up in the hot seasons and to move overland to fresh 

 waters. In the course of time they became more and more adapted 

 to spending part of their life on land, their paired fins actually de- 

 veloping into true legs. When this stage was reached they were no 

 longer fish, but primitive Amphibia. From some of these early Am- 

 phibia evolved the scaly reptiles, which are entirely independent of 

 water except for drinking, althougli some, like the crocodiles and 

 the turtles, have returned to an aquatic mode of life. Still later, a 

 branch of the reptiles gave rise to the mammals. Thus it will be 

 realized that it was the Rhipidistia which were our remote ancestors 

 (fig. 3). 



The second air-breathing group of fish apparently lacked the ability 

 to evolve further and became somewhat degenerate. They stayed in 

 their drying-up pools and just used their lungs to tide themselves over 

 until the next wet season. Nevertheless, by their ability to endure 

 unpleasant conditions they have so far avoided extinction. Lung- 

 fishes (Dipnoi) are still to be found in pools, swamps, and intermit- 

 tent rivers in parts of Australia, Africa, and South America. 



