CHARACTERISTICS OF VERTEBRATES. 345 



long Fishes first that separate vertebral bodies appear in 

 e supporting dorsal axis of the body ; (2) that as a 

 iracteristic, the vertebral column is less important than 

 another supporting rod, the notochord, which precedes the 

 vertebral column in the history alike of the race and of 

 the individual. Nor need we object to the popular title 

 backboned, provided we recognise that the adjective " bony " 

 is first applicable among Fishes, and not even to them 

 all. 



Characteristics of Vertebrata. 



(i) Dorsal Nerve-Cord. — In such Invertebrates as earth- 

 worm and crayfish, that is to say in Annelids and Arthro- 

 pods, the long ganglionated nerve-cord lies along the ventral 

 surface, and is connected anteriorly with the dorsal or 

 cerebral ganglia by a double commissure through which runs 

 the gullet. This is characteristic of Invertebrates. 



In Vertebrates, however, the central nervous system, con- 

 sisting of brain and spinal cord, lies on the dorsal surface, 

 and is tubular in structure. In Balanoglossus there is a 

 dorsal nerve-cord, and a ventral as well ; in Tunicates the 

 nerve-cord lies along the dorsal surface, but except in the 

 Appendicularia type it degenerates in the adults ; in 

 Amphioxus it persists, but there is hardly any brain ; in 

 higher Vertebrates the brain and spinal cord are well 

 developed. 



(2) Dorsal Axis or Notochord. — Beneath the ectodermic 

 spinal cord, in all young Vertebrates from Tunicates 

 upwards, there lies an endodermic supporting rod or noto- 

 chord. In Balanoglossus (Hemichordata) it is very slightly 

 developed, in the anterior region of the body ; in Tunicates 

 (Urochordata, i.e., with notochord in the tail), it is confined 

 to the posterior part of the body, and degenerates in the 

 adults except in the Appendicularia type ; in Amphioxus 

 (Cephalochordata) it persists and extends from the tip of the 

 head to the tip of the tail as a continuous rod ; in Cyclosto- 

 mata (hag and lamprey) it persists unconstricted as a 

 gristly rod ; in some Elasmobranchs and Ganoids the same 

 is true ; in all other Vertebrates it is constricted into seg- 



