ORIENTATION OF AMPHIOXUS 41 



ward swimming, are omitted. The whole locomotor response 

 is undoubtedly much more rapid during unimpeded progress, 

 hence it might be argued that although the animal in slower and 

 more deliberate swimming carried its anterior end persistently 

 in advance, yet when moving rapidly, if once reversed to back- 

 ward swimming (and I have shown above that this reversal 

 actually occurs from time to time), the physiological inertia of 

 the animal as it travelled at its highest speed would tend to keep 

 it so oriented until the swimming movements grew less energetic 

 and the animal returned to the deUberate swimming that is 

 characteristic at the end of the course. The data given at the 

 beginning of the paper, in which nine out of fifty animals were 

 recorded as oriented with the posterior end in advance while 

 swimming freely in large aquarium jars, would tend to strengthen 

 this suspicion; on the contrary, the actual results obtained from 

 a study of stained animals did not substantiate such a line of 

 reasoning. 



If an animal, with the exception of one end, is wrapped in a 

 fold of wet absorbent cotton and laid on a glass plate, the ex- 

 posed end can be immersed without difficulty in the stain; in 

 this case a weak solution of neutral red made up in sea-water 

 was used. The anterior end was the one stained in most cases, 

 for the more open structure of the pharyngeal region offers a 

 larger surface for the reception of stain. After the stain had 

 caused coloration to a deep pink or light reddish shade the animals 

 were allowed to recover over night. 



When stimulated after such treatment, the stained extremity 

 could be followed with comparative ease, and observations made 

 in this way corroborated my earlier conclusions. Amphioxus 

 does not locomote backward for any considerable distance, even 

 when the response is extremely vigorous; but, after a somersault 

 brings it tail-end in advance, either another reversal follows 

 directly or the animal changes its course and returns more or 

 less in the direction from which it came. 



I believe the observations recorded in the first experiment 

 of nine lancelets, which were supposed to swim backward, can 

 be explained as follows : When the swimming response is nearing 



