THE FROG AND VERTEBRATES IN GENERAL 157 



be found. It lies, as a rule, inactively on the bottom and attracts 

 other animals into its enormous mouth by means of a long, 

 waving dorsal fin ray. Another highly modified, bizarre Teleost 

 is the Porcupine-fish, which is a rather sluggish, bottom-living 

 type. At times, when disturbed, they are able to inflate them- 

 selves with the inhaled air so that they become almost globular 

 in shape. Tropical fishes, as found, for example, in the coral reefs 

 (page 53), and also many deep sea fish present an extraordinary 

 array of coloring and adaptations to their particular life condi- 

 tions. (W. f. 74.) 



Class IV. Amphibia. This vertebrate class includes the Cau- 

 data, such, for example, as Necturus and Amblystoma, shown in 

 figure 10 ; all of which are tailed forms. They are for the 

 most part aquatic and, in some cases, the gills are functional 

 throughout life. Most species, however, undergo metamorphosis, 

 the gills cease to function, and air-breathing lungs develop. The 

 Salientia, such, for example, as the Frogs and Toads, are all tailless 

 forms when fully mature. In the larval, or tadpole, stage they 

 are aquatic, tailed, fish-like organisms with functional gills and no 

 limbs. Then they metamorphose into air-breathing, adult individ- 

 uals which are markedly different from the tadpole in various ways, 

 but particularly in the absence of the tail and the presence of two 

 well-developed pairs of pentadactyl limbs. (W. fs. 76-78.) 



Thus the Amphibia are aquatic or semi-aquatic animals which 

 breathe by gills at all times or, more often, only during the larval 

 period. With very few exceptions the amphibian skin is smooth 

 and shows no exoskeletal structures, such as the scales of fishes or 

 of reptiles. Even more noteworthy are the pentadactyl limbs, 

 which mark a wide advance over the fish fin, and the development 

 of lungs. Inasmuch as the Frog is used later as a basis for the 

 description of, and laboratory studies on, the vertebrate organ- 

 ism, it will not be necessary to consider the Amphibia further at 

 this point. 



Class V. Reptilia. In this class, we encounter for the first 

 time a group of vertebrate animals which are air-breathing at all 

 stages in their life history. The embryonic gills are never func- 

 tional. The skin, in distinction to that of the Amphibia, is marked 

 by a considerable development of exoskeletal structures such as 

 are shown in the bony plates of the turtle or the scaly snake skin. 

 Undoubtedly the Reptiles reached their greatest development in 



