INTRODUCTION. 33 



Division IV. Vertebrata. 



The Yertebrate Division of the animal kingdom, as lias been already 

 remarked, includes animals with an internal articulated skeleton or 

 framework, capable of growth, sujjplied with blood-vessels and nerves, and 

 serving for the support of the soft parts ; and it is here that animals of the 

 greatest size are found. The body is divided into*head, trunk, and organs 

 of locomotion, and the nervous system has attained its greatest concentration 

 in a single brain or nervous centre. The group of Vertebrata having the 

 same value as the previous Divisions, it is unnatural to consider it as 

 balancing the Evertebrata conjointly, and on this account it will be here 

 considered as a division including the following classes. 



Class 1. Pisces. 



In all fishes the blood is oxygenated by means of gills, which are 

 supported by a bony framework, named the branchial arches, which 

 generally amount to four. The external structure is adapted for inhabiting 

 and moving through the water. The air-bladder, although not concerned 

 in breathing, is really the homologue of the lungs in the higher classes. It 

 is not present in all fishes. 



Fishes are divided into two series, according as the skeleton is 

 caHilaginous or osseous. In the former the organization is low, the ribs 

 are rudimentary, and in the lowest form tlie spine is a continuous line of 

 cartilage not yet divided into vertebrae. The orders of cartilaginous or 

 chondroiDterygeous fishes are as follows : 



Order 1. Cyclostomata. PI. 74, fig. 48, Petromyson., lampereel. 

 " 2. Selachii. " " 49, Squalus, shark. 



" 3. Sturiones. " " 51, Acipenser, sturgeon. 



Tlie osseous fishes, which are much the most numerous, are distributed 

 in the following orders : 



217 



