CHAPTER VIII. 



BEANCH VIII. VERTEBKATA. 



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 genuine brain; 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 vertebrae, 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 (vertebrae) of the spinal column. 



* Except in Amphioxus, in which the spinal nerves arise right and 



left alternately. 



