786 Comparative Morphology of Chordates 



larva and in adults of some permanently aquatic species. Otherwise it 

 is replaced by a stratum corneum. Mucous glands are of the complex 

 multicellular sort; bony scales are rare in modern amphibians. In 

 reptiles the stratum corneum is strongly developed and elaborated 

 into horny scales. The general integument is devoid of glands; bony 

 scales are lacking in most modern reptiles. Birds elaborate the stratum 

 corneum into feathers, lack integumentary glands (except the uro- 

 pygial oil-gland), and are totally devoid of bony scales. Mammals 

 derive their hair from the stratum cutaneum, lack integumentary 

 mucous glands but possess sweat-glands and oil-glands, and only 

 rarely have bony scales. An epidermal cuticula does not occur among 

 amniote vertebrates. In this series, beginning with fishes and cul- 

 minating in birds and mammals, the differences exhibited by the skin 

 are, in general, correlated with the nature of the environments of the 

 several groups. 



The notochord is functional in Cyclostomata. Among Pisces it 

 persists in a somewhat reduced way in adults of Chondrostei and 

 Dipnoi. Adults of other fishes retain only vestiges of it. In Amphibia 

 the notochord is usually functional during the larval stage but merely 

 vestigial in adults. Of Reptilia, the adult Sphenodon retains definite 

 remnants of the embryonic notochord. In other reptiles and in all 

 members of Aves and Mammalia, the notochord is restricted to the 

 embryonic stages. 



The embryonic skeleton (aside from the notochord) in all verte- 

 brates is cartilaginous. The degree to which it becomes ossified varies. 

 In Cyclostomata, Elasmobranchii, and Holocephali, the adult skeleton 

 (except the notochord) is entirely cartilaginous. In fishes of the "gan- 

 oid" groups, there is more or less ossification, and in Teleostei, the 

 "bony fishes," ossification reaches its limit for fishes. In modern 

 Amphibia, especially in the Urodela, the adult skeletons retain more 

 or less of the embryonic cartilage. In Reptilia ossification of the 

 skeleton is nearly complete (the sternum usually does not ossify), and 

 in Aves and Mammalia ossification reaches its extreme. 



The segmented muscles of the body exhibit varying degrees of 

 differentiation. Local elaboration of groups of muscles occurs in relation 

 to (1) the mechanism of breathing; (2)' the separation of an anterior 

 thoracic from a posterior abdominal region of the trunk; and (3) the 

 locomotor appendages. This differentiation is at its minimum in fishes. 

 Corresponding to the capacity of the lungs and to the locomotor 

 efficiency of the legs, there is some differentiation of appropriate 

 muscles in amphibians, a decidedly more elaborate differentiation in 

 reptiles, and in birds and mammals the extreme of complexity is 

 attained (Fig. 91). - 



