THE PHYLA HEMICHORDATA AND CHORDATA 



The order Apoda, limbless amphibians, is represented by a few small forms 

 found only in the warmer parts of the world. Examples are Ichthyophis, of 

 Ceylon (Fig. 18.15), and Siphonops, of South America (see Fig. 7.1, p. 215). 

 These amphibians are burrowing animals about the size of large earthworms. 

 Their developmental stages are passed in the burrow, within eggshells re- 

 sistant to desiccation, but the larval stages possess external gills and other 

 features characteristic of the aquatic stages of the Caudata and Salientia. 

 The limbless state of the Apoda is a highly specialized characteristic, but in 

 more important internal features they are among the most primitive of living 

 amphibians. 



The order Salientia contains the most familiar amphibians, the 'Reapers, " 

 comprising the many species of frogs and toads (Fig. 18.16). The line of 

 descent for this modern type was apparently established at the close of the 

 Age of Amphibians, but they were not clearly diflferentiated until the Age of 

 Reptiles was well advanced. Existing species of Salientia are widely dis- 

 tributed, and on the whole they are the most highly specialized of the modern 

 forms. Most species can live for long periods out of water if the atmosphere 

 is sufficiently moist, and some, such as the toad, Bufo amencanus, are more 

 nearly land animals than any other amphibians. Even toads, however, are 

 dependent on a moist atmosphere, and they return to ponds and streams dur- 

 ing their brief breeding season in the spring. The frogs are even more de- 

 pendent on water and a moist atmosphere, because of their greater use of the 

 damp skin in gas exchange. 



The general distribution of most amphibians is therefore conditioned by the 

 requirements of each species, particularly in connection with their "skin 

 breathing" and with their aquatic breeding habits. None of these forms is 

 truly a terrestrial animal like the reptile, which very often lives and breeds 

 in hot, dry places. 



The Reptilia. Reptiles are "cold-blooded," air-breathing vertebrates, 

 typically covered by horny epidermal scales and often with an underlying 

 armor of bony dermal plates. There are also characteristic but less con- 

 spicuous features of internal anatomy. In contrast with amphibians, which 

 are typically dependent on a moist atmosphere and water for reproductive 

 and developmental stages, reptiles are completely adapted for terrestrial life. 

 They live upon the land, even under desert conditions, and breed and un- 

 dergo embryonic development without dependence on water. Perfection of 

 lungs and air breathing, development of integuments and egg membranes 

 which resist desiccation, and certain modifications of embryonic development 

 are features that make possible this complete adaptation for terrestrial life. 

 Arising from amphibian ancestors, reptiles became the dominant land animals 

 of the Age of Reptiles and eventually gave rise to birds and to mammals. 

 The reptilian manner of development, as compared with those of amphibians 

 and mammals, represents a vital step in vertebrate e\olution. Most reptiles 

 are oviparous. The reptilian embryo, like the amphil^ian, develops in a 

 fluid medium, but its medium is the amniotic fluid (see p. 162); the protec- 



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