282 CHARiCTERS OF VERTEBRATE ANIMALS. 



Vertebrata, are united together by such forms as the Tunicates, 

 yet each have their own distinctive characters. 



On taking a transverse section of a vertebrate animal and 

 comparing it with the same of an invertebrate (fig. 147), we 

 shall note the following important differences. The section of 

 the invertebrate consists of a single tube in which lie the dorsal 

 hsemal system (H), the ventral nervous system {Nc), and the 

 central gut (Q). On comparing the vertebrate section, we 

 observe that there are two distinct tubes, in one of which, the 

 dorsal, is shut off the chief nervous system (iVe) ; in the other 

 lie the gut and the ventral hremal system — part of this latter, 

 the heart, being further shut off by the membrane surrounding 

 it, the pericardium (P). 



Vertebrates {Oraniota) may be said, generally speaking, to be 

 characterised by having, a dorsal nervous system which is en- 

 closed in a canal composed of a number of bony or cartilaginous 

 rings, the vertebrae, placed in a longitudinal row. In aU verte- 

 brates the structure called the notochord (iV) is present in the 

 embryo, and more or less so in the adult. This structure, 

 which is developed in the floor of the neural canal, is a gela- 

 tinous, cartilaginous, axial rod, tapering to a point at each end, 

 and forming a support to the cerebro-spinal canaL The noto- 

 chord, which is persistent in the AmpMoxus, is replaced in 

 true vertebrata by the vertebral column. This column is 

 composed of a series of cartilaginous or ossified segments, the 

 vertebrae, between which the primitive notochord becomes 

 squeezed out, it remaining as only small pads between the 

 vertebrae, or as diamond-shaped masses, as we see in the Fish. 

 The limbs of vertebrates are never more than four in number, 

 and are always turned away from the neural or dorsal surface ; 

 they are united to the body by distinct articulations. They 

 are often absent in the lower vertebrates (Snakes) ; in Fish 

 they are represented by the paired pectoral and pelvic fins (fig. 

 165, p and v). 



Although a vascular system was seen in the invertebrates, it 



