602 The Animal Kingdom 



considered as an amphibian. Captorhinus may be cited as the first true 

 reptile inasmuch as certain of its skull and skeletal characters are rep- 

 tilian rather than amphibian; whereas both Seymoiiria and Diadectes 

 had retained many amphibian characters. 



True land dwelling required the development of many different 

 seemingly unrelated characteristics : the scaled skin to prevent drying, 

 efficient lungs, complex changes in the circulatory system, an efficient 

 kidney, internal fertilization, and eggs that could be laid on land— just 

 to mention a few. Some of these characteristics were found among the 

 amphibians, but not until the reptiles were they all combined. 



The amphibian egg lacks protective envelopes to prevent drying, 

 thus must be laid in water. In contrast, the reptilian egg has not only 

 a protective shell, but also an amniotic sac in which the embryo develops. 

 Also there is a large amount of yolk, by which the embryo is nourished 

 and an allantois for respiratory exchange and excretion. Undoubtedly 

 these important adaptations must have appeared very early in some 

 aberrant labryinthodont ; unfortunately fossil records are absent, and 

 the exact time of its appearance is unknown. 



With but rare exceptions, the amphibians have external fertiliza- 

 tion. Obviously such a type of fertilization is impossible in land 

 dwellers; thus with the escape from water, internal fertilization was 

 required. 



Characteristics of the Class Reptilia.— Those characteristics 

 which distinguish the members of this class are to a great extent as- 

 sociated with terrestrial life. For the prevention of desiccation, they 

 are supplied with a dried cornified skin with scales. For respiration, 

 the lungs are well developed and divided into many alveoli ; thus with 

 only a few exceptions respiration is carried on only by the lungs. As- 

 sociated with this lung development, the circulatory system is greatly 

 improved over that of the amphibians. The heart has two auricles, and 

 the ventricle is almost completely divided into two chambers in most 

 forms, and in the crocodiles the separation is complete. The systemic 

 arches are reduced to a single pair, and the pulmonary system is well 

 developed. Fertilization is internal, and the eggs are provided with 

 embryonic membranes. In some species the eggs are retained within 

 the oviduct, and the young are born alive. Such a condition is known 

 as ovoviviparity. Whether oviparous or ovoviviparous, there are no 

 larval stages. Although the reptiles are poikilothermic animals (hav- 

 ing a variable body temperature) they do have a much more efficient 



