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THE AMERICAN NATURALIST [Vol. XL 



of these animals to the stimuli of their environment in the direction 

 of pedal locomotion on the sea bottom. It is highly probable 

 that these appendages arose as ventrally projecting bar-like struc- 

 tures, to enable the bottom-living forms (since all were bottom 

 dwellers to begin with) to move more readily from place to place 

 on the surface of the sea floor while remaining in contact with the 

 sea floor, thus avoiding the necessity of the more difficult feat of 

 balancing themselves in the lighter ambient fluid above the earth 

 floor, in the effort to effect a change of place. 



Paired appendages then did not arise as fins for the purpose of 

 balancing the animal in the water, but the paired fins of fishes have 

 been developed by the transformation of the primitive paired or- 

 gans of locomotion, of which the paired appendages of the Amphi- 

 bia and their descendants are the direct and, in their simplest 

 forms, the least modified derivatives. 



When one studies the life habits of amphioxus and Bdellostoma 

 in their natural element and, at the same time, the history of their 

 development, he no longer entertains the idea that these animals 

 have lost paired appendages once possessed by their ancestors, 

 but will, and can only, say that they are the ancestral forms of 

 animals possessing paired appendages, and that in the case of 

 amphioxus and of Bdellostoma we have two stages in the response 

 of the vertebrate stock to the stimulation of the environment 

 looking toward the formation of locomotor appendages. 



They are both bottom dwellers of necessity, although they take 

 occasional excursions into the superambient water, but quickly 

 fall back, from the force of gravity, to the bottom. These excur- 

 sions into the superambient liquid are effected by the motion of 

 the caudal fin from side to side. This fin is the main organ of 

 locomotion used by all fish-like vertebrates for progression through 

 the water. 



When amphioxus strikes the bottom after such an excursion, 

 it lies quietly upon its side, since it is unable to coil its body suf- 

 ficiently to lie on its ventral edge. Most of its life it passes buried 

 in the sand. To enter the sandy bottom in which it lives, it first 

 makes an excursion into the superambient water and then descends 

 head first upon the sandy bottom, boring its way among the parti- 

 cles of sand. W'hen swimming it maintains its body in the dorso- 

 ventral position. 



