2 Vertebrata. 



skeleton or support ; below l is a longitudinal body- 

 cavity, containing the organs of digestion, circulation, 

 respiration, &c. ; above is a second, smaller, longitu- 

 dinal cavity or canal, in which lie the brain and 

 spinal marrow, the central organs of the nervous 

 system ; these send out laterally along their whole 

 extent numerous pairs of nerve-cords to supply the 

 different parts of the body. Thus on cross-section 

 the body of a vertebrate animal appears like two 

 tubes, the smaller being above the larger, and the 

 cartilaginous axis appears in the middle of the 

 horizontal partition which divides them from each 

 other. 



In the young conditions of the tunicated worms 

 there is an approach to this arrangement, but in these 

 the gristly rod does not extend sufficiently far for- 

 ward to separate the neural (or nervous system- 

 holding) and visceral cavities. 



To the central axis of cartilage the name notochord 

 is given, and it is enveloped in a sheath of several 

 layers. In the majority of vertebrates the notochord 

 is present only as a temporary and transitory structure, 

 for, in the process of growth, parts of its sheath enlarge 

 and encroach on the axis itself, so as to obliterate 

 it eventually in whole or in part. These enlarge- 

 ments begin in the form of a succession of paired 

 lateral thickenings along the whole length of the 

 sheath, which extend above and below the notochord, 

 and become converted into rings around it, and 

 ultimately by extension inwards they become discs. 



1 The animal is supposed to be placed with its length hori- 

 zontal and its mouth forwards. 



