VERTEBRATES. 5 



especially Balanoglossiis, which exhibit structures suggestive 

 of affinity with Vertebrates. 



The Cyclostomata, represented by the lamprey {Fetro- 

 myzon) and the hag {Myxine), and some other forms, 

 probably including an interesting fossil known as Palao- 

 spondylus, are sometimes ranked with Fishes under the title 

 Marsipobranchii. But they have no definitely developed 

 jaws, no paired fins, no scales, and are in other ways more 

 primitive. 



The lancelet I^Amphioxus)^ for which the class Cephalo- 

 chorda has been erected, is even simpler in its general 

 structure. Thus there is an absence of limbs, skull, jaws, 

 well-defined brain, heart, and some other structures. The 

 vertebral column is represented by an unsegmented (or un- 

 vertebrated) rod, called the notochord, which in higher 

 animals (except Cyclostomata and some fishes) is a transitory 

 organ replaced by a backbone. 



The Tunicata or Urochorda form a class of remarkable 

 forms, the majority of which degenerate after larval life. 

 In the larvffi 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 (a 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 Balanoglossiis and its allies, for which the class Hemi- 

 chorda or Enteropneusta has been established, 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 struc- 

 ture on the anterior dorsal surface and other features, have 

 led some 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 Inverte- 

 brate forms. 



The distinction is a very old one, for even Aristotle distinguished 

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

 from cuttlefish, shell bearing animals, crustaceans, insects, &c., which 

 he regarded as " bloodless." He was, indeed, mistaken about the 

 bloodlessness, but the distinctiveness of the higher animals first men- 

 tioned has been recognised by all subsequent naturalists, though it was 

 first precisely expressed in 1797 by Lamarck. 



