GENERAL CHARACTERS OF THE VERTEBRATA, 217 
column. They all possess a notochord, but this may remain 
persistent throughout life, and in many cases the development 
of the spinal column is very imperfect. 
The skeleton of all Vertebrate animals is zz¢erna/, and the 
muscles are attached to its several parts. The value of this 
character is in no way affected by the fact that many Vertebrates, 
such as the Tortoises, Crocodiles, and others, possess an ex- 
ternal skeleton as well. The “més of Vertebrate animals are 
always articulated or 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 two pairs, and they always have 
an internal skeleton for the attachment of the muscles of the 
limb. 
A distinct blood-vascular or ‘‘heemal” 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 modifica- 
tions 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 
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 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 
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 cranzuim 
orskull. 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 “ verte- 
bree,” and they have the following general structure. Each 
