166 STRUCTURE OF THE VERTEBRATES 



motor apparatus when the vertebrates left the water and 

 muddy flats and became true land living animals. 



H. Specialization of the Limbs 



Modifications of the appendages begin with the fish, the two 

 main lines of evolution having been traced, that is, the typical 

 balancing fin, and the crossopterygian type. The amphibia arose 

 as five-toed animals with short legs, and there are few speciali- 

 zations in the class except the loss of digits and entire limbs. 

 Thus, in the recent amphibia, we have species without append- 

 ages, others lacking the hind legs, one with minute front and 

 hind legs, and some lacking some of the digits. The typical 

 amphibian has five digits on the pelvic limb, and four on the 

 pectoral. 



Among the reptiles the specializations become more pro- 

 nounced. The snakes and some lizards have no limbs, the pythons 

 having minute stubs on the degenerate pelvic girdle; and others 

 lack certain digits. Among the fossil reptiles there are many 

 adaptive radiations, many of which are parallelled in the mam- 

 mals and may be considered under separate headings as func- 

 tional adaptations. The student should keep in mind that these 

 similar evolutionary changes are separate lines of specialization, 

 and that there is no genetic relationship between the modifica- 

 tions in different classes. The typical five toes of the fossil 

 amphibia and reptiles were inherited by the mammals, and any 

 specializations for particular function have arisen independently. 



1. Aquatic. Numerous orders of reptiles and mammals show 

 varying adaptations for water life. Among the reptiles, the 

 turtles (page 58) show many marked changes of the append- 

 ages. In addition to the shift in shape of the carapace (shell) 

 the appendages become webbed, and in the sea-turtle the feet 

 become flippers. The aquatic life was carried to its furthest ex- 

 tent in the extinct ichthyosaurs (fish-like lizards). These rep- 

 tiles had flippers with the digits completely enclosed by the skin 

 covering, the body was fusiform in shape, the tail was short 

 with a terminal flattening, and the animals were ovo-viviparous 

 (page 37) . Apparently they lived their entire lives in the water. 



The mammals, recent and fossil, show similar adaptations. 



