PRIMITIVE VERTEBRATES. y) 
The Tunicata or Urochorda are remarkable forms, the 
majority of which degenerate after larval life (Fig. 8). 
In the larvee of all, and in a few adults which are neither 
peculiarly specialised nor degenerate, we recognise some of 
the fundamental characters of Vertebrates. Thus there is a 
dorsal supporting axis (or notochord) in the tail region, a 
dorsal nervous system, gill - clefts 
opening from the pharynx to the 
exterior, a simple ventral heart, and 
so on. 
Of Balanoglossus and its allies 
(Hemichorda or Enteropneusta) it is 
still difficult to speak with confidence. 
The possession of gill-clefts, the 
dorsal position of an important part 
of the nervous system, the occurrence 
of a short supporting structure on 
the anterior dorsal surface of the 
pharynx, and other features, have led 
many to place them at the base of 
the Vertebrate series. 
Characteristics of Vertebrata.—At 
this stage, having reached the base of the . 
Vertebrate series, we may seek to define a Fic. 8.—Ascidian or 
Vertebrate animal, and to contrast it with sea - squirt, — After 
Invertebrate forms. Haeckel. 
The distinction is a very old one, for 
even Aristotle distinguished mammals, birds, reptiles, amphibians, 
and fishes as ‘‘ blood-holding,” from cuttle-fish, 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 expressed in 1797 by Lamarck. 
Yet it is no longer possible to draw a boundary line between Verte- 
brates and Invertebrates with that firmness of hand which characterised 
the early or, indeed, the pre-Darwinian classifications. We now 
know—(1) that Fishes and Cyclostomata do not form the base of the 
Vertebrate series, for the lancelets and the Tunicates must also be in- 
cluded in the Vertebrate alliance ; (2) that Balanoglossus, Cephalodiscus, 
and some other forms, have several Vertebrate-like characteristics ; 
(3) that some of the Invertebrates, especially the Chzetopod worms, 
show 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. 
