GENERAL CIIAUACTEIiS OF THE VEETEBKATA. 



219 



Leaving the brain and spinal conl out of siglit for a moment, we see 

 tliat the lower or visceral tube of a Vertebrate animal contains tlie 

 digestive canal (/;), the hiemal or blood-vasculai- system (c), and a 

 system of nervous ganglia (/(). Now this is exactly what is con- 

 tained within the visceral cavity of any Invertebrate animal ; and 

 it follows from this that the " cerebro-spinal " nerve-system of the 

 Vertehrata is a structui'e for which no pi-ccise ])arallel exists among 

 Invertebrates. 



Another peculiarity wliicli is present in all the Vertebrata is, that 

 at an early period of life there is develojied in the liiwer wall of the 

 tube which contains the cerebro-s|iinal axis a siugidar structure 

 known as the " notochord," or "chorda doj'salis" (hg. 157, B, cli). 

 This is a semi-gelatinous rod, tajiering at both ends, an<l extending 

 along the floor of the cerebro-spinal tulie. In some cases the note- 

 chord remains permanently in this condition, but in most cases it is 

 replaced at maturity by the bony colunui or backbone, from which 

 the Vertahrata derive their name. The general structure of the 



Fi". Vu. — D!;i^,'raiiis i I'jpTX'Si'iifiii,^^ transM'isi; scr) ions nf nnc 



° A— anil uue of tlir Veiii'lij-ahL, H. ri W'M >,( the li.«l 



Htemal or blooil-vascnlar system : 7/ XeT\nHs system 



brain and spinal cord, enclosed in a separate tube ; 



dorsalis. 



if the hi-hm- Inwrlebrata, 



' ; Ij Alinn'ntar}' eanal ; <; 



II Cerebin-spinal axis or 



fh Xiituehord, ur chorda 



vertebral cohimn will be ilescribed shortly, and it is sufficient to 

 state here that it consists of a series of bony or cartilaginous seg- 

 ments or " vertebne," arranged so as to foi-ju a longitudinal axis 

 upon which the spinal cord is supported. It is to lie remembered, 

 however, that all Vertebrate animals do not possess a vertebral 

 column. They all possess a notochord, but this may remain per- 

 si.stent throughout life, and in many cases the development of the 

 spinal column is very imperfect. 



The skeleton of all Verteljratc aidmals is iiiti'mul, and the nuiscles 

 are attached to its sever.al ]i;irts. The value of this character is in 

 no way affected by the fact tliat ni:i,iiy Verteljrates, such as the 

 Tortoises, Crocodiles, and othci's, pcjsscss an cit'-ni'il skeleton a.s 

 well. The liiuha of Verleliratc ;iiiiiii:\ls are j'.lways articulated or 



