PRIMITIVE VERTEBRATES. 



animals (except Cyclostomata and some fishes) is a tran- 

 sitory embryonic organ afterwards replaced by a backbone. 



The Tunicata or Urochorda form a class of remarkable 

 forms, the majority of which degenerate after larval life. 

 In the larvae of all, and in the 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, for 

 which the class Hemichorda or 

 Enteropneusta has been established, 

 it is still difficult to speak with 

 confidence. The possession of gill- 

 clefts, the dorsal position of an im- 

 portant 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 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 lancelet 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 ChEetopod worms, 

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



Fig. 8. — Ascidian or 

 sea - squirt. — After 

 Haeckel. 



