CHAPTER FORTY-THREE 



The noto- 



chord 



Dorsal 

 nerve cord 



PROCHORDATA AND CYCLOSTOMES 



I. THE vertebrates are distinguished from inverte- 

 brates by the possession of a vertebral column. That 

 is to say, they possess a so-called backbone, which 

 consists of a great number of bones, the vertebrae, 

 arranged in a series. Prior to the development of this 

 structure, in the very early embryo of all vertebrates, 

 appears a rodlike element known as the notochord. 

 It is not cartilage, and does not become bone, but it 

 occupies the place of the subsequently developing verte- 

 brae, and has an essentially similar function, that of 

 stiffening the animal. 



In all vertebrates the main nerve cord is dorsal ; 

 that is to say, it is on the upper rather than the lower 

 side of the animal, being just above the notochord. 

 In invertebrates the reverse is true, so that we may say 

 that the orientation or position of the vertebrates is 

 reversed as compared with the invertebrates. 



Breathing Vertebrates breathe in different ways, terrestrial forms 

 gills 16 an d aquatic ones derived from them (as whales) having 



lungs, while primitively aquatic groups possess gills. 

 The gills are, however, very different from those of 

 invertebrates in the majority of instances, although the 

 function of absorbing oxygen from the water is the 

 same. The young of the lowest types of fishes, and 

 even the adults of certain amphibians (such as the 

 Necturus or mud puppy), possess external gills, which 

 correspond in general structure to those of many 

 invertebrates. We find, however, that in adult fishes 

 there is another type of gill, which consists essentially 

 of an arrangement whereby water, entering through 

 the mouth, passes out on each side through the gill 



320 



