312 CHAKACTEKS OF VEETEBKATE ANIMALS. 



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

 yet each have their owti distinctive characters. 



On taking a transverse section of a vertebrate animal and 

 comparing it with the same of an invertebrate (iig. 180), we 

 shall note the following important differences. The section of 

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

 haemal system (H), the ventral nervous system (iVc), and the 

 central gut ((?). 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 heemal system — part of this latter, 

 the heart, being further shut off by the membrane surrounding 

 it, the pericardium (P). 



Vertebrates {Craniota) 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 all verte- 

 brates the structure called the notochord (N) 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 at each end, and 

 forming a support to the cerebro- spinal canal. The noto- 

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

 vertebrata by the vertebral column. This column is composed 

 of a series of cartilaginous or ossified segments, the vertebrse, 

 between which the primitive notochord becomes squeezed 

 out, it remaining as only small pads between the vertebrse, 

 or as diamond-shaped masses, as we see in 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. 

 198, p and v). 



Although a vascular system occurs in the Invertebrates, it 



