VERTEBRATES 



13 



name Chordata, under which the Vertehrata are now commonly rated a 

 subphylum. 



The primary subdivisions of this subphylum was first made by 

 Aristotle, who recognised four groups, namely, fishes, oviparous 

 tetrapodous and apodous vertebrates (amphibians and reptiles), 

 birds, and viviparous vertebrates (mammals). Linnaeus recognised 

 the same four groups and named them classes in his system, giving 

 them the modern names of Pisces, Amphibia, Aves and Mammalia, 

 but he did not unite them in a single phylum. In Cuvier's system 

 the same classes were also recognised, but for the name of the second 

 class the term Reptilia was employed in place of Amphibia. This 

 system was maintained in the classification and the amphibians 

 and reptiles placed in a common class until about the year 1850, when 

 the French zoologist Milne-Edwards called attention to the fundamental 

 subdivision of all vertebrates into two great groups, the Anamia and 

 the Amniota, and also to the fact that inasmuch as the Amphibia 

 belong to one and the Reptilia to the other of these groups these 

 two subdivisions of the Vertebrata should each constitute an indepen- 

 dent class. Of the five classes of vertebrates thus formed the English 

 naturahst Huxley in 1871 formed three divisions, the Ichthyopsida 

 or fish-like vertebrates, the Sanropsida or reptile-like vertebrates 

 and the Mammalia. 



Classification. — About 33,400 species of vertebrates have been 

 described, grouped in 5 classes, of which about 12,000 species are 

 fishes, 1,400 are amphibians, 3,500 are reptiles, 13,000 are birds and 

 3,500 are mammals. The primary grouping of these classes gives 

 the following subdivisions: 



A. Anamia or Ichthyopsida. — Vertebrates whose eggs are deposited 



in most cases in the water and are without a shell; and whose 

 embryos are protected only by an albuminous envelope and the 

 water and not by an amnion; which respire by means of gills 

 either during the whole or a part of life; and which have a slimy 

 epidermis lacking special epidermal coverings (Fishes and 

 Amphibians) . 



B. Amniota. — Vertebrates whose eggs (in the oviparous species) 



are laid on the land and have the protection of a shell; whose 

 embryos have the additional protection of an amnion and respire 

 by means of an allantois; which respire after birth by means 

 of lungs; and have special epidermal coverings in form of scales, 

 feathers or hair. 



