CHARACTERS OF VERTEBRATES 



is preceded and sometimes, as in the lower fishes, co- 

 exists with the un jointed skeletal rod, the notochord 

 whose presence characterizes the entire phylum Chordata. 

 They breathe too by means of perforations fringed with 

 gills which put the front end of the alimentary tract in 

 communication with the outside world, or at least 

 possess in the embryo traces of these breathing organs. 

 The gill slits of fishes are these organs and more will 

 be said concerning them on a later page. The verte- 

 brata moreover possess a skull, a series of bones sur- 

 rounding the brain ; they have also two pairs, and only 

 two pairs, of limbs, which, however, may partly or 

 entirely disappear (e.g. snakes, some lizards, etc.). 

 These limbs are provided with a skeleton. The central 

 organ of impulsion of the vascular system, the heart, 

 is always ventral in position. No animal that is not a 

 vertebrate combines these characters in itself. 



These necessarily technical details now require some 

 expansion and explanation. All the vertebra ta possess 

 a brain and a spinal cord, running along the back, and 

 lying within the bony or gristly tube known as the 

 vertebral column. The brain is enclosed in an expanded 

 bony or gristly box, the skull. It is only in certain 

 lowly organized fishes, such as the sharks, that the skull 

 and vertebral column is entirely made of gristle or 

 cartilage ; in the higher types this cartilage becomes 

 bone, more completely so in the highest types, such as the 

 mammalia, and less completely so in many lower types, 

 such as the frog. In no creature belonging to the different 

 phyla, which were originally grouped together under the 

 name of Invertebrata, is there such an axial skeleton 

 enclosing the whole of the central nervous system. It is 

 commonly and loosely said, that in vertebrates the 

 skeleton is internal, and in invertebrates if there be a 

 sketeton it is external. That this is not strictly : true 

 is shown by the external skeleton in the shape, of scales 



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