10 VERTEBRATE ANIMALS. 



eight classes. These are: 1. MAMMALIA, or Mammals; 

 2. AVES, or Birds; 3. REPTILIA, or Reptiles^ 4. BATRACHIA, 

 or Batrachians; 5. PISCES, or True Fishes; 6. ELASMO- 

 BRANCHII, or Selachians; 7. MARSIPOBRANCHII, or Myzonts; 

 8. LEPTOCARDII, or Lancelets. 



Of these classes, two, Elasmobranchii and Leptocardii, 

 are represented by marine forms only, and do not, 

 therefore, come within the scope of this treatise. 



The relations of the classes of Vertebrates may be 

 recognized by the following analysis, taken, in part, 

 from Prof. Gill's "Arrangement of the Families of 

 Fishes." Only the most obvious characters are here 

 referred to, although others, less striking, are often of 

 greater taxonomic value. 



CLASSES OF VERTEBRATES. 



* Respiration never performed after birth by means of branchiae. 



f Exoskeleton developed as hair (rarely obsolete) ; warm blood ; 



heart with four cavities ; diaphragm complete ; two occipital 



condyles ; viviparous ; young developed from a minute egg, 



and nourished for a time by milk secreted in the mammary 



glands of the mother. .... MAMMALIA. 



ft Exoskeleton developed as feathers ; warm blood ; heart with 



four cavities; diaphragm incomplete; a single occipital 



condyle ; oviparous ; young hatched from a comparatively 



large egg ; no mammary glands AVES. 



tff Exoskeleton developed as scales, or bony plates ; cold blood ; 



heart with three cavities (four in CrocodilicC) ; a single 



occipital condyle ; oviparous (or rarely ovoviparous) ; young 



hatched from a rather large egg. . . . REPTILIA. 



** Respiration performed by gills for a part, or the whole, of life ; 



cold blood. 



\ Skull more or less developed, with the notochord not continued 

 forwards beyond the pituitary body ; brain differentiated 

 and distinctly developed; heart developed and divided 

 into at least an auricle and ventricle. 



