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CHAPTER I. 



CHARACTERS OF THE SUB-KINGDOMS, 



SUB-KINGDOM I. Vertebrata. 



9. The first primary division of animals is termed Verte- 

 brata. It is so called on account of the almost universal 

 presence of a vertebral column or backbone. The 

 skeleton or hard parts are internal ; some groups, how- 

 ever, such as the Crocodiles and Tortoises, have also an 

 external skeleton. This skeleton consists of the bony 

 case, called the cranium or skull, and a series of pieces, 

 which are united together, and form the axis, termed the 

 vertebral column. Attached to the vertebrae there is 

 generally a number of bony arches, termed ribs, which 

 are united by cartilages to the sternum or breastbone. 

 Then there are the bones that support the limbs, 

 which are greatly modified in the various groups. 



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PIG. l. 



A, Transverse section of an invertebrate animal ; B, Section of a vertebrate : 

 b, wall of the body ; a, alimentary canal ; h, haemal or blood system ; n, principal 

 nervous system; n', sympathetic nervous system of vertebra ta; c, notochord. 

 It will be seen, that, in the Vertebrata, the main mass of the nervous system is 

 dorsal (Lat dorsum, the back), and ventral (Lat. venter, the belly) iu the 

 Iiivertebrata, while the blood system is ventral iu the Vertebrata, and dorsaj 

 in the Invertebrata, 



