CHAPTER XXXVIII. 



THE VERTEBRATA, PISCES, AND AMPHIBIA. 



THE vertebrata are distinguished from other forms of animal 

 life by the possession of an internal skeleton. The principal part 

 of the skeleton is the vertebral column composed of several dis- 

 tinct bones called vertebrae. Each vertebra is made up of a body 

 and several processes or projections. Some of these processes en- 

 able the vertebrae to articulate with one another so as to form an 

 elongated, flexible, bony axis. Other processes jut out from each 

 vertebra so as to form a narrow bony tube, or canal on the dor- 

 sal side of the axis, and at least the framework of a much larger 

 cavity on the ventral. And perhaps we find here the chief differ- 

 ence between the two groups of animals : the in vertebrates consist 

 of onebody cavity and its contents; the vertebrates havethesame, 

 and in addition a second cavity, the vertebral canal and its con- 

 tents. While the Vertebrata are alike in many respects they differ 

 widely among themselves, forming five quite well-marked classes : 

 Pisces or Fishes, Amphibians, Reptiles, Aves or Birds, and Mam- 

 malia. 



The class Pisces is made up of cold-blooded aquatic animals 

 that breathe by means of gills. As usually constituted, this class 

 includes several forms that are not regarded as true fishes. Of 

 these the lowest in the scale are the Leptocardii. They have a 

 membrane-cartilaginous skeleton, without a skull, the vertebral 

 column represented by an elastic substance called the notochord, 

 which in the higher vertebrates represents the column in the em- 

 bryo. The colorless blood circulates in pulsating vessels without 

 the aid of a heart; they have no nervous system or sense organs. 

 The mouth without jaws leads into the pharyngeal sac, whose 

 sides are pierced by numerous slits which serve for respiration. 

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