27 



THE CLASSIFICATION AND STRUCTURE 



OF VERTEBRATES 



In earlier chapters we have described the dogfish, frog, pigeon, 

 and rabbit, which are the types of vertebrate most often studied 

 in elementary courses of zoology, but for a proper understanding 

 of the way in which the vertebrate body is built we need to know 

 something about other animals ; in particular there is a big gap 

 between the dogfish and rabbit which is only partially filled by 

 the frog, an aberrant and unimportant creature which is dissected 

 chiefly because it is (or was) cheap and plentiful. In this chapter 

 we shall consider the chief organ systems in turn, showing their 

 range in complexity and presumed course of evolution, but first 

 we must have a classificatory framework to which to refer. 



The Vertebrata or Craniata, the fourth subphylum of the 

 Chordata (p. 309), are distinguished from the others by the 

 possession of a bony or cartilaginous skeleton, part of which is 

 arranged as a jointed dorsal longitudinal axis, the vertebral 

 column or backbone, consisting of separate pieces called vertebrae ; 

 at the front end of this is a skull which partially or entirely 

 encloses the brain. Characteristic features of the soft parts are 

 the great development of the brain, and with it of the organs 

 of sight, smell and hearing ; a heart with at least one receiving 

 and one pumping chamber ; the respiratory pigment haemoglobin 

 in corpuscles ; a small number of gill slits (seldom more than 

 ten pairs) ; and an excretory system based on the coelomoduct 

 (p. 189). The notochord is usually lost in the adult and never 

 extends to the anterior tip of the body. Most vertebrates have 

 paired limbs, and possibly all the earlier forms had them, but 

 as there are now several groups which are limbless this is not a 

 very practical distinction. 



Superclass I — Agnatha 



The earliest known fossil vertebrates, from the Silurian and 

 Devonian periosd, have no jaws, and are placed in a separate 

 super-class. The only similar living animals are a few semi- 

 parasitic genera called lampreys and hagfishes, which are without 



542 



