CHAPTER XVIII 



INTRODUCTION TO THE PHYLUM VERTEBR ATA, SUB-PHYLA. 

 HEMICHORDA, CEPHALOCHORDA AND UROCHORDA 



THE Vertebrata comprise almost all the larger animals, including 

 Man. The name simply means jointed (Lat. vertebra, 

 of?he Phylum. a ji n ^ anc ^ especially a bone of the spinal column), 

 and refers to the possession of a jointed internal axis 

 as the main part of the skeleton. In the lowest forms this axis 

 is not developed, but in place thereof there is a smooth elastic 

 rod, which has received the name of notochord, literally back- 

 string (Gr. vwrov, back ; x / ^ string). In all the members of the 

 phylum this notochord is present at some stage of development, 

 although in the higher forms it subsequently becomes surrounded 

 and obliterated by the jointed rod or vertebral column. Hence the 

 name Chordata, which has been proposed for the group, is really 

 more appropriate ; but as the term Vertebrata" has been sanctioned 

 by long usage it is inadvisable to depart from it. 



Besides the possession of the notochord there are two other 

 features by which the Vertebrata are distinguished. One of these 

 is that all Vertebrata possess at some period of their lives slits in 

 the wall of the front part of the alimentary canal. In the lower 

 and more simply organised members of the phylum these slits allow 

 the water which is taken in at the mouth for purposes of respiration 

 to escape, and hence they are called gill -slits. The other feature 

 common to all Vertebrata 'is that the main mass of the nervous 

 system takes on the form of a dorsal strip of sensitive skin the 

 medullary plate, which becomes wholly or partly enrolled to 

 form a tube, the neural canal or spinal cord. 



There are in all about 32,000 known species of Vertebrata, 

 including all the more familiar animals fish, frogs, reptiles, birds 

 and mammals; so that the word animal to the ordinary mind 

 s. & M. 25 



