A HISTORY Of VERTEBRATES: FISHES 443 



bottom-dwelling life. Like the skates and rays, they are flattened and 

 glide along the bottom with up and down undulatory movements. But 

 instead of being flattened dorso-ventrally, they are greatly compressed 

 from side to side, and swim turned over on one side (Fig. 22.13 B). 

 During larval development, the eye that would be on the "ventral" side 

 migrates to the top surface, but the mouth does not change position. The 

 skates and flounders present a good example of convergent evolution, 

 by which animals that are widely separated in the evolutionary scale 

 independently adapt to similar modes of life. They acquire similar 

 adaptive features, in this case a flattened body shape, though in diffier- 

 ent ways. 



Other teleosts have adapted to a life among seaweeds and in coral 

 reefs. The sea horses with their monkey-like, prehensile tails; the sar- 

 gassum fish with its camouflaging color and weedlike protuberances; 

 and the elongate, snakelike moray eels are examples (Fig. 22.13). 



A few teleosts have adapted to life in the ocean depths. Such fish 

 often have light-producing knninescent organs, presumably for species 

 recognition, and large mouths and greatly distensible stomachs to take 

 full advantage of the occasional meal that may come their way. 



Some teleosts live in intimate association with other fishes. The 

 remora has an anterior dorsal fin that is modified as a suction cup and 

 is used to attach to sharks. It feeds upon crinnbs of the larger fish's 

 meals, or obtains free rides to favorable feeding grounds. Relationships 

 of this type, in which one organism benefits and the other receives 

 neither benefit nor harm, are known as commensalism (Fig. 22.13). 



A few teleosts have become amphibious. The Australian mudskip- 

 per frequently hops about on the mud flats of mangrove swamps at low 

 tide in search of food, and may even bask in the sun. It has unusually 

 muscular pectoral fins to help pull itself along the land, and it can close 

 its opercular chamber and extract oxygen from the air with its gills. 



Many other fascinating adaptations are found among these fishes, 

 but we must not dwell upon them for the teleosts are only a side issue 

 in the total picture of vertebrate evolution. The main branch toward 

 the higher vertebrates passed through the less spectacular Sarcopterygians 

 of ancient Devonian swamps. 



Fleshy-Finned Fishes. Sarcopterygian evolution diverged at an 

 early time into two lines— the lungfishes (order Dipnoi) and the crossop- 

 terygians (order Crossopterygii). The primitive crossopterygians were 

 the less specialized, having a well ossified internal skeleton and small 

 conical teeth suited for seizing prey. It is from this group that the am- 

 phibians arose. Lungfishes early in their evolution developed specialized 

 crushing tooth plates, and showed tendencies toward reduction of the 

 internal skeleton and paired appendages. In certain other features lung- 

 fishes and crossopterygians have paralleled actinopterygian evolution. 

 They evolved symmetrical tails, though of a type that is symmetrical 

 internally as well as externally (diphycercal), and their primitive, thick, 

 bony scales, which were characterized by having a thick layer of dentin- 

 like cosmin, have tended to thin to the cycloid type. 



