CHAPTER LVI 

 REPTILES AND BIRDS 



The next two classes, which include reptiles and birds, have so 

 many features in common that it has been suggested that they form a 

 single class, the Sauropsida. This view, however, has not been generally 

 accepted. These classes differ from Amphibia by characteristics which 

 show a more decided adaptation to terrestrial life and which completely 

 emancipate the animals included in them from an aquatic environment. 

 Although there are in each class types that have returned to aquatic 

 life, they do not again regain the characteristics which belong to aquatic 

 vertebrates as such. 



407. Structural Characteristics. — Among the structural character- 

 istics which the reptiles and birds possess in common and which separate 

 them from the amphibians are: (1) They possess but one occipital con- 

 dyle for articulation of the cranium with the vertebral column; the 

 amphibians have two. A condyle is a rounded projection with an 

 articulating surface. (2) A complete thoracic basket is formed by the 

 ribs, which meet a sternum, or breastbone, in the ventral median 

 Hne. (3) Respiration is carried on throughout life by lungs, and though 

 branchial arches appear early in embryonic hfe, their development ceases 

 before gill shts are formed. (4) The kidney is a metanephros. (5) The 

 eggs are meroblastic and not holoblastic. (6) Embryonic membranes 

 known as the amnion and allantois are developed during embryonic life. 

 Reptiles and birds also differ from mammals in the following ways: (1) 

 The latter have two occipital condyles. (2) In mammals the lower jaw 

 articulates directly with the cranium. In other vertebrates with bony 

 skulls there is a quadrate bone interposed in this articulation. The 

 quadrate bone is thought to have disappeared from its original position 

 in this articulation by becoming one of the three bones incorporated in 

 the middle ear of mammals. (3) Mammalian development shows 

 characteristic modifications, adjusting the young to development within 

 the body of the mother. 



408. Embryonic Modifications. — The eggs of fishes and of amphibians, 

 which are laid in the water and buoyed up by it, are usually protected 

 only by a gelatinous covering. The aquatic environment prevents them 

 from drying and they do not suffer from the effects of mechanical con- 

 tacts since they move freely in the water in which they are suspended. 

 When, however, as in the case of the reptiles and birds, the eggs are 

 deposited outside water, they need protective envelopes to prevent 



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