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THE AMERICAN NATURALIST [Vol. XL 



be said, however, to possess the power of locomotion except by 

 swimming or springing. Undoubtedly the next step in the trans- 

 formation of the Bdellostoma-like body of the ancestral Gnath- 

 ostome was the gradual formation of pedal appendages, which 

 enabled it to move easily, certainly, and symmetrically over the 

 bottom. 



It is just as erroneous to maintain any hypothesis which would 

 derive the paired appendages of the Amphibia, for example, from 

 the paired fins of fishes as it is to maintain the claim that the 

 pectoral appendages (arms — forelegs) of land vertebrates are 

 derived from the wings of birds. 



When we consider the structure of the nervous system we are 

 again brought face to face with the fact that amphioxus represents 

 a developmental stage in the central nervous system repeated by 

 other members of the vertebrate stock. Its nerve cord possesses 

 all the relations to the other main organs of the body that are pos- 

 sessed by the central nervous system of other vertebrates. It 

 lies immediately above the dorsal surface of the notochord in the 

 hollow skeletal tube composed of a connective tissue membrane, 

 in the walls of which, however, no chondroidal tissue is formed, 

 and in which no calcareous matter has at any time been deposited, 

 but this condition of the protective tube of the central nervous 

 system is reproduced in the development of all the other verte- 

 brates, from the amphioxus to man. This tube is not surrounded 

 by skeletal structure in the amphioxus, but it is perforated with 

 lateral openings made through its lateral face for the exit of the 

 nerves passing out from it and entering it. In this it is also in 

 harmony with the conditions found in all other vertebrates. 



In the antero-posterior direction, the nerve tube is divided 

 into two main parts, as in all other vertebrates, a brain and a 

 spinal cord. While there are differences of histological value 

 between the spinal cord of amphioxus and higher forms, this dif- 

 ference is hardly greater than exists between species of the higher 



Wv may distiiiss further consideration of this part of the nervous 

 system with the statement that it is in every respect a vertebrate 

 spinal cord. When we come to consider the structure of the 

 brain, however, we find a simplicity in the arrangement of the 



