CHARACTERS OF VERTEBRATES 
is preceded and sometimes, as in the lower fishes, co- 
exists with the unjointed 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 vertebrata 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 soin the highest types, such as the 
mammalia, and less completely so in many lower types, 
suchasthefrog. Innocreature 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 
3 
