VERTEBRATE AND INVERTEBRATE 5 



beneath the nerve-tube, is a slender elastic rod which acts as 

 a primitive skeleton. This is the notochord, which in higher 

 forms is more or less obliterated and replaced by the backbone 

 or vertebral column. The main skeleton of vertebrates is 

 internal and not outside the body as in many invertebrates. 



The higher forms have appendages, either fins or limbs, 

 four in number arranged in two pairs. They are composed 

 of tissue derived from several segments, not from one only as 

 in invertebrates. 



The differences and similarities between this fundamental 

 plan and that of invertebrates may conveniently be set out in 

 tabular form. 



Vertebrates and most higher Invertebrates agree in being : 



bilaterally symmetrical ; 



coelomate ; 



metamerically segmented. 

 Vertebrates differ from Invertebrates in having : 



a notochord ; 



a dorsal and tubular nerve-cord ; 



gill-slits ; 



a postanal tail ; 



a ventral heart through which blood flows forwards ; 



main skeleton internal ; 



appendages formed from several segments ; 



a hepatic portal system ; 



dorsal and ventral nerve-roots. 



