CHAPTER V 
THE SYSTEMATIC POSITION AND CLASSIFICATION OF FISHES 
In the first chapter of this volume it was pointed out that the 
Craniata, of which the Fishes form a subordinate group, is 
the last of the four principal divisions into which the Chordata 
are divided. The animals included in the first three, viz. the 
Hemichordata, the Urochordata, and the Cephalochordata, have 
already been dealt with in the earlier chapters, and it now 
remains for us briefly to consider the diagnostic characters of 
the Craniata, and then, more in detail, the organisation of the 
Fishes. 
The Craniata, often termed Vertebrata, form one of the best 
defined and most easily recognisable divisions of the animal 
kingdom. As the name implies, they are distinguished from the 
more primitive Chordata by the formation of a definite “ head,” 
as the result of the modification of the anterior portion of the 
central nervous system to form a complex brain, round which are 
concentrated the chief organs of special sense. This is combined 
with the evolution of a skull, which, in addition to providing a 
“cranium ” for the enclosure and protection of the brain, and partial 
or complete capsules for the sense-organs, is connected behind with 
a system of bony or cartilaginous visceral arches, which loop 
round the pharynx between the gill-clefts. Besides supporting 
the breathing organs (gills) in the lower aquatic Craniata, or 
existing as embryonic vestiges in the higher lung-breathing forms, 
these arches usually form the basis of jaws for the mouth. The 
epidermal portion of the superficial skin is always composed of 
several layers of cells. The notochord, which is always present 
in the embryo, and in a few Craniates, both living and extinct, 
may even be retained in its entirety in the adult, fails to reach 
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