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. 



405. 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 condyle at the 

 base of the cranium; the amphibians have two. A condyle is a rounded 

 projection for articulation with the vertebral column. (2) The lower 

 jaw on each side is connected with the cranium by means of a quadrate 

 bone, a bone derived from the first branchial, or hyoid, arch. (3) A 

 complete thoracic basket is formed by the ribs, which meet a sternum, 

 or breastbone, in the ventral median line. (4) Respiration is carried on 

 throughout life by lungs, and though branchial arches appear early in 

 embryonic life, their development ceases before gill slits are formed. 

 (5) The kidney is a metanephros. (6) The eggs are meroblastic and not 

 holoblastic. (7) 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 condyles. (2) In mammals the quadrate bone 

 does not enter into the articulation of the lower jaw but becomes one 

 of the bones of the middle ear, the lower jaw itself articulating with the 

 cranium. (3) Mammalian development shows characteristic modifica- 

 tions, adjusting the young to development within the body of the mother. 



406. 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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