xxxii Introduction. 



tion, is exactly reversed in the Vertebrate; and the former may 

 X consequently be spoken as ' neuropodous/ and the latter as ' haema- 



podous/ In both cases the digestive tube interposes itself for 

 ^ greater or smaller distances between the haemal and neural systems, 

 but the perforation of the nerve-system, by the anterior segments 

 of the digestive tube, which constitutes the nerve-collar of Inverte- 

 brata, finds no representation in the relations subsisting between 

 the principal or cerebro-spinal nerve-centres of Vertebrata and 

 their digestive tube, the anterior or oral opening of which is always 

 directed towards the ventral, and away from the neural surface 

 of their bodies. The perivisceral cavity of Vertebrata never com- 

 municates with the blood-vascular, though it has recently been 

 shown to communicate with the lymphatic system ; nor is it ever 

 prolonged into their limbs, which possess always an internal and 

 segmented skeleton either of cartilage or of bone. The limbs of 

 Vertebrata differ further from the limbs of Arthropoda in never ex- 

 ceeding the number of two pairs. Externally, the Vertebrata show 

 no appearance of segmentation, and the segmentation which they do 

 exhibit internally does not affect the organs of vegetative life, but 

 is exemplified only in their skeleton, nerves, and muscles. The 

 axial portion of the internal skeleton, which separates the body of 

 the Vertebrate animal into a neural and haemal cavity, is not 

 always divided by segmentation into the structures whence the 

 Sub-kingdom takes its name. In the Amphioxus, the endo-skeleton 

 is represented simply by the rod-like aggregation of cells known as 

 the chorda dorsalls ; by the sheath surrounding this structure ; and 

 by fibrous arches, which are developed above and below in con- 

 nection with these axial structures, and give attachment laterally 

 to the inter- muscular septa ; but it shows no other signs of segmen- 

 tation except by the possession of series of mesially-arranged carti- 

 laginous nodules, which correspond in position to the inter-spinous 

 bones and fin-rays of more highly organized fish. In all other 

 Vertebrata, the endo-skeleton becomes definitely segmented poste- 

 riorly to the head, either by the development of cartilaginous neural 

 arches alone, as in Petromi/zon, or by the development of axial, in 

 addition to neural indurations. By the more or less perfect fusion, 

 and, ordinarily, by the calcification of these elements, the structures 

 known as ' vertebrae ' are formed. 



In all Vertebrata, with the exception of Amphioxus, which is 



