APPENDIX B 



661 



majority of the Paleozoic "stegocephalians" (*Labyrinthodontia, Figs. 27.17 and 

 19, so named because of the complex infolding of the enamel layer of their large 

 teeth), and the modern frogs (Fig. B.13) and toads (Figs. 15.8 and 9) (Salientia or 

 Anura). 



Subclass 2. The Lepospondyli have "husk vertebrae" in which the centra are 

 formed by deposition of bone directly around the embryonic notochord. Includes 

 a few groups of small Paleozoic stegocephalians and the modern orders Urodela 

 (salamanders, Fig. B.31) and Apoda (blindworms or caecilians). The salamanders 

 are lizardlike in form; as adults some are aquatic while others live in moist ter- 

 restrial habitats. The blindworms are small, burrowing, wormlike tropical animals; 

 some are oviparous, the male possessing a copulatory organ, unlike most 

 amphibians. 



Fig. B.31. Representative amphibians. Left, the grass frog, Rana pipiens; right, ihe cave 

 salamander, Eurycea lucifuga. {Courtesy Ward's Natural Science Establishment, Inc., and 

 American Museum of Natural History, respectively.) 



Class 6. The Reptiles (Reptilia). Lizards, snakes, turtles, alligators and croco- 

 diles, Sphenodon, and numerous *extinct groups of the Paleozoic and Mesozoic. 

 Cold-blooded, fully terrestrial vertebrates (except for secondary adaptations to 

 aquatic existence), without gills at any time; independent of the water for breed- 

 ing, through possessing a shelled egg along with internal fertilization; oviparous 

 or ovoviparous; skin with few glands, covered by horny epidermal scales unlike 

 the dermal scales of fishes, making it dry and waterproof; with two pairs of 

 pentadactyl limbs except when these have been lost through degeneration, the 

 toes usually armed with claws — the forerunners of the claws, hoofs, and nails of 

 birds and mammals; lung breathing aided by ribs; heart three-chambered, the 

 ventricle partly subdivided, thus tending toward the four-chambered condition 

 and complete double circulation; third eyelid, pineal eye, and depressed eardrum 

 developed; skull with a single median occipital condyle. The classification of the 

 reptiles has been dealt with in detail in Chap. XXVIII. 



Class 7. The Birds (Aves). Warm-blooded, air-breathing vertebrates, with 

 primary adaptations for flight; body covered with feathers; forelimb three-fin- 

 gered, modified into a wing (in a few groups it is secondarily specialized for 

 swimming, or vestigial) ; heart four-chambered, with complete double circulation, 

 single right aortic arch preserved; respiration by means of highly perfected lungs, 

 their capacity increased by accessory air sacs that penetrate the coelomic cavity 

 and the interior of the bones, reducing relative weight and thus aiding in flight; 

 all modern birds with a syrinx or voice-organ situated at the junction of the 



