220 VERTEBRATE AKIMALS. 



jointed to the body, and they are always turned away from that 

 side of the body (the " neural " side) upon which the great masses 

 of the nervous system are placed. The limbs may be altogether 

 wanting, or partially undeveloped, but there are never more than 

 tii:o pairs, and they always have an internal skeleton for the attach- 

 ment of the muscles of the limb. 



A distinct blood-vascular or " haemal " system is present in all 

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

 contractile cavity or heart, furnished with valvular openings. 



Lastly, the masticatory organs of all Vertebrates are modifications 

 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 in the Invertebrates. 



The above are the leading characters which distinguish the Ver- 

 tebrata 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 ^'^ertebrates, commencing with their bony frame- 

 work or skeleton. 



The skeleton of the Yertehrala may be regarded as consisting 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 regai-ded as essen- 

 tially 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 " vertebral column.'' The segments which form the vertebral 

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

 structure. Each vertebra (fig. 158, 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 vertebrje are all united together in different 

 ways, so as to give the column great flexibility. From the back of 

 the body of the vertebra proceed two bony arches which unite 

 behind and thus form with the centrum a bony canal in which the 

 spinal cord is contained. For this reason these arches [n) are called 

 the " neural " arches. From the point where the neural arches 

 unite — that is to say, from the back of the neural canal — proceeds a 

 long process, sometimes cleft at its extremity, termed the " spinous 

 process " (s). Springing also ^rom each neural arch is a second 

 sliorter process {a) termed the " articular process," since by means 

 of these, as well as by the bodies, the vertebrfe are jointed or "ar- 



