VERTEBEATA 



TTERTEBRATA, the name of a great branch or phylum 



V of the Animal Kingdom which comprises those ani- 



La- mals having bony " vertebra ", or pieces of bone jointed 



marck's go as to form a spinal column. The first recognition of 



* ncl . , the group is due to Lamarck (1797), who united the four 



Cuvier s , . , . . , . , 



classifi- highest classes of Linnreus s system as ammaux a verte- 



cations. bres," whilst distinguishing the rest of the animal world as 

 "animaux sans vertebres." The same union of the four 

 Linnsean classes had been previously made by Katsch in 

 1788, who, however, proposed for the great division thus 

 constituted the name " Knochenthiere." The significance 

 of Lamarck's classification was materially altered, and the 

 foundation laid of our present attempts to represent by 

 our classifications the pedigree of the animal kingdom, 

 when Cuvier propounded his doctrine of " types," and re- 

 cognized the Vertebrata as one of four great types or plans 

 of structure to be distinguished in the animal world (i). 1 



The Vertebrata of Lamarck and Cuvier included beasts, 

 birds, reptiles, and fishes, and until recently the group 

 was considered as one of the most sharply limited in the 

 animal kingdom. The progress of anatomical studies 

 very soon rendered it clear that all Vertebrata did not 

 possess bony vertebras ; for, besides the commoner sharks 

 and skates, with their purely cartilaginous skeletons, natu- 

 ralists became acquainted with the structure of fishes, such 

 as the sturgeons and the lampreys, which possess no verte- 

 brae at all, but merely a continuous elastic rod (the noto- 

 chord) in the place of the jointed spinal column. The 

 muscles and their skeletal septa were seen in these fishes 

 to be arranged in a series of segments attached to the sides 

 of this continuous rod ; and hence the structural character 

 of bony vertebra;, as distinguishing the Vertebrata, gave 

 place to the character of segmental arrangement of the 

 muscles of the body-wall, such muscles being supported 

 by a skeletal axis which might be itself unsegmented 

 (notochord), or replaced by segmental cartilaginous or bony 

 Essential vertebra. The studies of embryologists furnished a sound 

 struc- foundation for this conception by demonstrating that in 

 hires of t j ie eul bryos of Vertebrata with true vertebrae these struc- 

 Verte " tures are preceded by an unsegmented continuous noto- 

 teS ' chord. The inquiry into the structural characteristics of 

 Vertebrata led further to the recognition of several addi- 

 tional points of structure, the combination of which was 

 present only in the group which had been recognized by 

 Lamarck on superficial grounds^Jt was found that all 

 T These numerals refer to the bibliography at the end of the artiek-. 



Vertebrata possess laterally-placed passages leading from 

 the pharynx to the exterior, serving in the aquatic forms 

 as the exits for water taken in by the mouth, and provided 

 with vascular branchial processes, whilst in the embryos 

 of the higher air-breathing classes they appear only as 

 temporary structures. It was further established that the 

 great mass of nervous tissue lying dorsally above the 

 spinal column, and known as the cerebro- spinal nerve- 

 centre or brain and spinal cord, is in all cases a tube, 

 and originates as part of the dorsal surface of the embryo, 

 which becomes depressed in the form of a long groove and 

 finally closed in by the adhesion of its opposite edges, thus 

 forming a tube or canal. The three structures, noto- 

 chord, gill-slits, and tubular dorsal nerve-cord, were more 

 than twenty years ago recognized as characterizing, together 

 with the metameric segmentation of the musculature of 

 the body-wall, all Vertebrata at some one or other period 

 of their existence. 



The establishment by Darwin of the doctrine of organic Question 

 evolution in 1859 led naturalists consciously to make the?' Verte - 

 attempt to determine the genetic affinities and the probable eestry ' 

 ancestry of the various groups of animals, and enabled 

 them to recognize in the classifications by " type ", and 

 other such conceptions of earlier systematists, the uncon- 

 scious striving after genealogical representation of the 

 relationships of organic beings. The question naturally 

 arose in regard to the Vertebrata, as in regard to other 

 great divisions of the animal kingdom, What were the 

 characters of the earliest forms, the ancestors of those now 

 living 1 ? Then came the further questions as to whether 

 any surviving Vertebrata closely resemble the ancestral 

 form, and whether any animals are still in existence which 

 retain the general characters of those primeval forms 

 which were the common ancestors at once of Vertebrates 

 and of other large and equally well-marked phyla or 

 branches of the animal kingdom, such as the Molluscs, the 

 Annulates, &c. This fascinating subject of inquiry received 

 its most important impulse from the embryological investi- 

 gations of the Russian naturalist Kowalewsky, and has 

 been for nearly a quarter of a century the fertile source of 

 speculation and its indispensable accompaniments, new 

 observation and research. Kowalewsky published in 1866 Kowa- 

 an account of the embryology of the lowest and simplest '"vxkv's 

 of then recognized Vertebrates, the lancelet (Amphioj-ns laljours ' 

 lanceolatus), in which he attempted to trace, cell for cell 

 from the fertilized egg-cell, the origin of the characteristic 



