PRIMITIVE VERTEBRATES. 



some fishes) is a transitory embryonic organ afterwards 

 replaced by a backbone. 



The Tunicata or Urochorda are remarkable forms, the 

 majority of which degenerate after larval life (Fig. 8). 

 In the larvae 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. 



At this stage, having reached the base of 

 the Vertebrate series, we may seek to define 

 a Vertebrate animal, and to contrast it with 

 Invertebrate forms. 



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 distinctive- 

 ness of the higher animals first mentioned has been recognised by all 

 subsequent naturalists, tin nigh it was first precisely expressed in 1797 

 by Lamarck. 



Vet 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 (i) t-hat Fishes and Cyclostomata do not form the base of the 

 Vertebrate series, for the lancelet and the Tunicates must also be in- 

 cluded in the Vertebrate alliance ; (2) that Balanoglossus, Cephalodiscns, 

 and some other forms, have several Vertebrate-like characteristics : 

 (3) that some of the Invertebrates, especially the Cha^topod worms, 

 show some hints of affinities with Vertebrates. The limits of the 



FIG. 8. -- Ascidian or 

 sea - squirt. After 

 Haeckel. 



