GENERAL CHAEACTEES OF THE VERTEBEATA. 197 



The skeleton of all Vertebrate animals is internal, and tte 

 muscles are attached to its several parts. The value of this 

 character is in no way affected by the fact that many Verte- 

 brates, such as the Tortoises, Crocodiles, and others, possess 

 an external skeleton as well. The limhs of Vertebrate ani- 

 mals are always articulated or jointed to the body, and they 

 are always turned away from that side of the body (the " neu- 

 ral " side) upon which the great masses of the nervous system 

 are placed. The limbs may be altogether wanting, or partial-, 

 ly undeveloped, but there are never more than two pairs, and 

 they always have an internal skeleton for the attachment of 

 the muscles of the limb. 



A distinct blood-vascular system is present in all Ver^ 

 tebrates, and in all except one — the Lancelet — there is a 

 single contractile cavity or heart, furnished with valvular open- 

 ings. 



Lastly, the masticatory organs of aU Vertebrates are modi- 

 fications of parts of the walls of the head, and are never 

 modified limbs or hard structures developed in the mucous 

 membrane of the digestive tube, as they are ia the Inverte- 

 brates. 



The above are the leading characters which distinguish the 

 Vertebrata as a whole, and, before going on to consider the 

 different classes, it may be as well to give a short and general 

 sketch of the anatomy of the Vertebrates, commencing with 

 their bony framework or skeleton. 



The skeleton of the Vertebrata may be regarded as con- 

 sisting of the bones which go to form the trunk and head on 

 the one hand, and of those which form the supports for the 

 limbs on the other hand. The bones of the trunk and head 

 may be regarded as essentially composed of a series of bony 

 rings or segments, arranged longitudinally. Anteriorly, these 

 segments are much expanded and also much modified to form 

 the bony case which encloses the brain and which is termed 

 the cranium or skull. Behind the head, the segments enclose 

 a much smaller cavity in which is contained the spinal cord, 

 and they are arranged one behind the other, forming the " ver- 

 tebral column." The segments which form the vertebral 

 column are called " vertebrae," and they have the following 

 general structure : Each vertebra (Fig. 94, A) consists of a 

 central portion known as the "body," or "centrum" (c), 

 placed immediately below the spinal cord, and giving origin to 

 certain " processes." The ends of the bodies of the vertebree 

 are all united together in different ways, so as to give the col- 



