PRE-CAMBRIAN AND PALEOZOIC ERAS 163 



We may note that amphibians have never completely conquered the ter- 

 restrial environment. Their method of locomotion on land is inefficient. 

 They are not provided with skins which prevent undue loss of water from 

 the body by evaporation. And perhaps most important of all, most of 

 them must return to the water to lay their eggs. A few have developed a 

 variety of expedients to avoid returning eggs to the water, but none of 

 these expedients hold promise of general usefulness, as did the method 

 developed by the first reptiles (pp. 168-169). 



In the water amphibian eggs develop much as do fish eggs, and the 

 aquatic larvae, "tadpoles," have many of the characteristics of fishes, thus, 

 incidentally, affording an example of recapitulation (p. 50). Interestingly 



nostril 



adhesive 

 gil[s organ 



FIG. 8.20. Amphibian tadpole exhibiting external gills. 



enough, the "external gill stage" of an amphibian larva (Fig. 8.20), the 

 stage in which branching, frondlike gills project laterally from the surface 

 of the head, is similar to stages in the development of some modern 

 remnants of ancient groups of fishes: Polypterus, and some of the Dipnoi, 

 lungfishes. If we ever learn of the larval development of the Crossopterygii 

 we shall probably find that the latter had an "external gill stage" too. 



Mississippian Period 



The Mississippian period and the one following it have frequently 

 been regarded as subdivisions of one period called Carboniferous. The 

 Mississippian period is frequently called Lower Carboniferous, the Penn- 

 sylvanian period Upper Carboniferous. The name "Carboniferous" re- 

 fers to the formation of coal. In the Mississippian, however, little or no 

 coal was formed. 



Extensive limestone deposits of this period were formed principally, not 

 from coral reefs, for corals were relatively scarce, but from vast num- 



