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; G. Elasmo- 

 BKANCHii, or Selachians; 7. Marsipobranchii, or Myzonts; 

 8. Leptocardii, or Lancelets. 



Of these classes, two, Elasniohranchii 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 jDart, 

 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 g<^^, 

 and nourished for a time by milk secreted in the mammary 

 glands of the mother. .... jVIammalia. 



ff Exoskeleton developed as feathers; warm blood; heart with 

 four cavities; diaphragm incomplete; a single occipital 

 condyle; oviparous; young hatched from a comparatively 

 large ^^^g ; no mammary glands Avp:s. 



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

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

 occipital condyle; oviparous (or rarely ovoviparous) ; young 

 hatched from a rather large ii2.g. . . . Reptilia. 



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

 cold blood. 



t ^kull more or less developed, with the notochord not continued 

 forwards beyond the pituitary hody ; brain differentiated 

 and distinctly developed; heart developed and divided 

 into at least an auricle and ventricle. 



