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. Tliis 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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