No. 470] 



UNITY OF GNATHOSTOME TYPE 



83 



In the case of the Bdellostoma we have a very different con- 

 dition. The Bdellostoma possesses a body remarkable for its 

 flexibility and its elasticity, and is fitted not only to swim in a 

 dorso-ventral position, but during life, when it rests upon the 

 bottom, always does so with a dorso-ventral orientation. While 

 amphioxus shows a distinct inability to orient itself and main- 

 tain equilibrium in this position for any but the shortest periods 

 of time, Bdellostoma exhibits in a high degree the capacity to 

 maintain its position with ease, accuracy, and for an indefinite 

 period. This capacity for the equilibration of its body without 

 the possession of paired appendages is both remarkable and de- 

 serving of more careful study than has yet been given. 



With the possession of a simple ear, the tubular portion of 

 which lies in one plane of space, and with the lack of equilibrating 

 paddles in the form of pectoral and pelvic appendages, Bdellostoma 

 maintains its position while in motion with at least as great pre- 

 cision and as great apparent ease as any of the vertebrates possess- 

 ing paired appendages. 



It is clear, then, that it is not the necessity for the equilibration 

 of the body that has brought about the development of the paired 

 appendages in the vertebrate stock. 



Since the formation of the paired appendages in all other water 

 dwellers besides fishes is for the purpose of moving the body over 

 the bottom, that is locomotion, it becomes very ])r()l>al)lo that the 

 fish fin is a secondary structure, derived from the primitive })e(lal 

 appendages, which were used bv the ancestral form ex( lusivelv 

 for locomotion. 



This effectively disproves the theory of Gegenbaur of an 

 ancestral archipterygium, and the Tliatcher-Balfour lateral fin- 

 fold theory, and it removes from the field of vertebrate m()rj)li()logy 

 one of the most diflficult problems which it has been eaUe.! upon 

 to solve, by simplifying the conditions of the problem. 



While Bdellostoma is able to swim with ease, accuracy, and 

 precision, and thus change its position in space either for the 

 purpose of capturing prey, avoiding its enemies, or seeking a 

 new position upon the bottom, it is not able to progress upon the 

 bottom except by using the same swimming motion of its caudal 

 region which enables it to progress through the water. It cannot 



