PARKER. — THE SENSORY REACTIONS OP AMPHIOXUs. 435 



they were placed in a floating aquarium the sides of which were of 

 netting so as to permit a strong current of sea water to pass through it, 

 they either drifted to the far end of the aquarium or swam irregularly 

 about and without reference to the current, though a few small fish 

 that were caught and put into the aquarium swam against the current 

 with precision. 



These observations are in agreement with what Lyon (:05) found as 

 to the rheotropism of certain fishes, namely, that in large general cur- 

 rents their orientation is dependent not upon the direct stimulus of 

 the current, but upon the possession of a visual organ capable of form- 

 ing an image whereby they could fix their position in reference to mo- 

 tionless objects on the banks and in the bed of the stream. Since 

 amphioxus does not possess visual organs of such a character, orienta- 

 tion under these circumstances is not to be expected. 



If, however, an amphioxus is put into a large cylindrical vessel filled 

 with sea water and the water is made to whirl in it, the animal is quickly 

 stimulated to swimming and .swims vigorously against the current. 

 After a short period of active swimming, in which the animal will often 

 progress more rapidly than the current moves in the opposite direction, 

 it will drop to the bottom as though exhausted and be carried round 

 and round by the water. It was evident from the movements of the 

 animal that the stimuli to its locomotion were the momentary contacts 

 with the inner sides of the vessel next which it was often swept and in 

 all probability the varying rates of those parts of the current that 

 touched the sides of the animal. To such an irregular current amphi- 

 oxus undoubtedly reacts, i. e., under these circumstances it is rheotropic. 



Amphioxus can also be shown to be slightly geotropic. This fea- 

 ture does not appear in its swimming, for though Steiner ('86, p. 498 ; 

 '88, p. 43) affirms that the whole animal, or even a quarter of it, will 

 swim with full equilibrium, and is so quoted by Ayers ('92, p. 318) 

 and by Sherrington ('99, p. 1276), my own observations agree with 

 the statements of Rice ('80, p. 8) and of Willey ('94, p. 10), that in 

 swimming amphioxus may move with any side uppermost and may 

 continually change that side. This change of attitude during loco- 

 motion was so constant a phenomenon among the many amphioxus 

 that I watched that there is not the least question in my mind that 

 this animal during locomotion assumes no uniform position in reference 

 to gravity. 



In its resting state, however, amphioxus shows some slight response 

 to gravity. As it lies on the sand it may rest for considerable periods 

 of time with any side uppermost, but after it has burrowed and come 

 to rest near the surface of the sand, it usually lies, as Rice ('80, p. 8) 



