MAMMALIA 61 



Segmentation, or division of the body into a number of similar 

 parts or segments from before backwards, is another characteristic 

 of Vertebrates, but is not equally obvious in all of them. It is 

 particularly well seen in the lancelet (Amphioxus), a small some- 

 what fish-like animal classed with the Protochordates (fig. 293). 



The body of an average Vertebrate is divisible into head, 

 trunk, and tail, though the last may be entirely absent in some 

 terrestrial forms, as, e.g., the frog, which, however, possesses a 

 large tail in its tadpole stage. When limbs are present they are 

 never more than four in number, in which case the fore- and hind- 

 limbs may be much alike, as in a pig, lizard, or newt, or else 

 they may be widely dissimilar, as in a bat or bird, where the 

 fore-limbs are modified for the purposes of flight. One pair of 

 limbs may be entirely 

 absent, as in whales, 

 which possess no hinder 

 extremities, though there 

 can be no doubt that 

 such were present in the 

 ancestral forms from 

 which whales are de- 

 scended. The complete 

 absence of limbs in cer- 

 tain cases may be due 



. , Fig. 34. Left side (apparent dorsal side) of a Turbot 



to the fact that they 



never have been present, as in Protochordates; or it may be a 

 case of the dwindling away and disappearance of members once 

 possessed, as in snakes, which occasionally retain small and 

 imperfect hind-limbs. 



The double-tube arrangement, which has been described as 

 characteristic of the human body, is found among Vertebrates 

 generally, and running along the axis of the trunk between the 

 two tubes is either a backbone proper, or at any rate a firm sup- 

 porting rod which answers the same purpose. A rod of this 

 sort, the notochord, is found throughout life in the lancelet, for 

 example. It is not composed of bone or gristle, but of an elastic 

 substance. Such a rod is found in the embryos of all Vertebrates, 

 but as development proceeds it is usually more or less replaced 

 by a vertebral column made of cartilage or bone, or both. The 

 presence of a notochord at some period or other of life is one 



