CHAPTER XV. 



CHARACTERS AND AFFINITIES OF VERTEBRATES. 

 Vertebrata — " Backboned" Animals. 



The distinction between Vertebrates and Invertebrates is a 

 very old one, for even Aristotle distinguished mammals, 

 birds, reptiles and amphibians, and fishes as " blood-holding," 

 from cuttlefish, shell-bearing animals, crustaceans, insects, 

 etc., which he regarded as " bloodless." He was, indeed, 

 mistaken about the bloodlessness, but the distinctiveness of 

 the higher animals first mentioned has been recognised by 

 all subsequent naturalists, though it was first precisely ex- 

 pressed in 1797 by Lamarck. 



Yet it is no longer possible to draw a boundary line 

 between Vertebrates and Invertebrates with that firmness of 

 hand which characterised the early or indeed the pre- 

 Darwinian classifications. For we now know — (i) that Fishes 

 and Cyclostomata (hag and lamprey) do not form the base 

 of the Vertebrate series, but that the lancelet {Amphioxus), 

 and the sea-squirts (Tunicata) must be included in the 

 Vertebrate alliance ; (2) that a yet simpler worm-like form, 

 Balanoglossus, has several essentially Vertebrate character- 

 istics; (3) that some of the recognised Invertebrates especially 

 Chsetopods and Nemerteans, exhibit some hints of affinities 

 with Vertebrates. The limits of the Vertebrate alliance 

 have been widened, and though the recognition of their 

 characteristics has become more definite, not less so, the 

 apartness of the sub-kingdom has disappeared. 



It does not matter much whether we continue (with Ray 

 Lankester) to retain the familiar title Vertebrata, or adopt 

 that of Chordata, provided that we recognise — (i) that it is 



