270 SYSTEMATIC ZOOLOGY 



is a corresponding segmental arrangement of nerve fibers given 

 off from the spinal corcl. The segments on the two sides of the 

 body are not opposite one another, but alternate in position. 

 The tendency at the present day is to regard Amphioxus as 

 closely related to the following subtype, and as we have already 

 noted, some zoologists place it within that group. 



SUBTYPE IV. VERTEBRATA 



The Vertebrata ( Lat. vertebratus, jointed) comprise what are 

 commonly called the higher animals in distinction from those 

 which we have considered up to this point, which are often 

 called the Invertebrata (Lat. in, without, or not, and vertebratus, 

 jointed). In addition to the general characteristics which the 

 Vertebrata possess in common with the other Chordata, they 

 always have some kind of an axial skeleton developed about 

 the notochord. This skeleton consists of segmentally arranged 

 parts, the vertebrae, and comprises a cranium, which incloses 

 the brain, and the vertebral column or backbone, which gen- 

 erally incloses the neural tube. In the course of the develop- 

 ment of the vertebral column the notochord may persist, or it 

 may be partially or wholly obliterated. The central nervous 

 system always has the form of a tube, which is modified at its 

 anterior end to form a brain. 



The paired lateral appendages of the Vertebrata never exceed 

 two pairs, and two pairs are present in most cases ; the more 

 anterior pair is known as the pectoral, the more posterior as 

 the pelvic. These appendages are likewise provided with a 

 skeleton, called the appendicular skeleton. The blood consists 

 of a nearly colorless fluid in which there are colorless cells, the 

 white blood corpuscles, and also colored cells, commonly called 

 the red blood corpuscles; to these latter the color of the blood 

 is due. Reproduction in the Vertebrata is always sexual. This 

 group is divided into six classes, the members of which are 

 known commonly as fishes, amphibians, reptiles, birds, and 

 mammals. 



CLASS I. CYCLOSTOMATA 



The Cyclostomata (Gr. kvkXos, circle, and a-ro/xa, mouth) con- 

 sist of the lamprey eels and the hag-fishes or slime eels. While 



