PARKER. — THE SENSORY REACTIONS OF AMPHIOXUS. 441 



foremost and that he never saw it move with its tail in advance. 

 Steiner ('86, p. 497 ; '88, p. 41) also asserts that the animal moves with 

 the anterior end foremost. The locomotion of amphioxus is a rapid, 

 curiously irregular wriggle, often accompanied with somersault-like 

 movements which make it impossible to be sure at any moment 

 whether the animal is swimming backward or forward. The results of 

 momentary stimulation, however, show very conclusively that amphi- 

 oxus can swim both backward and forward, and that the direction of 

 swimming at the beginning of any course is dependent upon the part of 

 the animal's body that was stimulated. But how long amphioxus 

 keeps to one form of movement I was unable to discover. The fact 

 that it usually buries itself in the sand tail first (p. 433) leads me to 

 believe that, though it can swim forward, as maintained by Rice and by 

 Steiner, it usually swims backward. 



Another feature of the reactions of amphioxus is their great energy, 

 which is quickly followed by what seems to be complete collapse. For 

 a few moments the animal swims with the utmost vigor, and then drops 

 down quite motionless, as though it had become entirely exhausted 

 (Rice, '80, p. 9). That this is not exhaustion is seen from the fact that 

 a slight stimulus will usually cause a second round of activity ; but 

 after a few such efforts, the animal becomes unresponsive to further 

 stimulation and is doubtless temporarily exhausted. 



7. Central Nervous System and Sensory Mechanisms 



in Amphioxus. 



To what extent the uninjured central nervous system of amphioxus 

 is essential to its sensory reactions has already been briefly alluded to 

 in the account of this animal's reactions to light (p. 424), but now that 

 the other classes of stimuli have been described a more extended dis- 

 cussion of this subject may be undertaken. Steiner ('86, p. 498 ; '88, 

 p. 43), who was apparently the first to investigate the functions of the 

 central nervous system in amphioxus, states that after an animal had 

 been cut into two, three, or even four parts, all the parts reacted to 

 mechanical stimulation by swimming forward, and from these observa- 

 tions he concluded that the central nervous system of amphioxus is a 

 metameric structure without sufficient differentiation to allow one to 

 divide it into brain and spinal cord. Although his description of the 

 reactions of the pieces of amphioxus might lead one to infer that these 

 fragments reacted exactly as the whole animal did, it is plain from his 

 further account that such fragments were less sensitive than when they 

 made a part of the whole animal ; for he goes on to remark that, when 



