CHAPTER VIII. 
—_—— ’ 
BRANCH VIIIL—VERTEBRATA. 
General Characters of Vertebrates.— The fundamental 
characters of the Vertebrates are the possession of a 
segmented vertebral column, enclosing a nervous cord, and 
a skull which contains a gerne Sans yet these features, 
though common to most Vertebrates, are wanting in the 
lancelet (Amphioxus) and in a degree in the hag-fish, and 
even the lamprey ; but the essential character is the division 
of the body-cavity by the notochord (in the lancelet, etc.), 
or by the back-bone of higher Vertebrates into two sub- 
ordinate cavities, the upper (neural) containing the nervous 
cord, and the lower (enteric) the digestive canal and its ap- 
pendages and the heart. These are the only characters which 
will apply to every known Vertebrate animal (compare p. 206 
with Figs. 366, 370, and 371). 
In general, however, the Vertebrates are distinguished 
from the members of the other branches by the following 
characters: they are bilaterally symmetrical animals, with a 
dorsal and ventral surface, a head connected by a neck with 
the trunk; with two eyes and two ears, and two nasal open- 
ings, always occupying the same relative position in the head ; 
an internal cartilaginous or bony, segmented skeleton, con- 
sisting of vertebre, from the bodies of which are sent off 
dorsal processes which unite to form a cavity for a spinal 
cord, the latter sending off spinal nerves in pairs * correspond- 
ing to the segmentations (vertebra) of the spinal column. 
* Except in Amphioxus, in which the spinal nerves arise right and 
left alternately. 
