432 PROCEEDINGS OF THE AMERICAN ACADEMY. 



cumulated debris. This reaction is doubtless dependent upon the 

 mechanical stimulation caused by the particles of sand, etc., on the 

 cirri, for, as already stated, the momentary contact of the end of 

 the bristle with the cirri will call it forth. 



The great sensitiveness of the anterior end of amphioxus, which has 

 already been noticed by Krause ('88, p. 146), is resident chiefly in the 

 outer surface of the oral hood. This part of the animal is easily stimu- 

 lated by contact with any moving body and is the region especially 

 concerned with the reception of stimuli when, through the movements 

 of a few individuals, a whole assembly is set in violent commotion. 

 It is also probable that this part is especially stimulated when an 

 amphioxus, almost buried in sand, is made to draw back under the 

 sand by directing a fine stream of water on the exposed anterior end. 



In the middle-trunk region the firm dorsal and lateral walls, and 

 even the delicate ventral one, are relatively insensitive to mechanical 

 stimulation. 



The whole of the caudal region is more sensitive to mechanical 

 stimuli than the trunk region, but less so than the anterior end. 

 The surface about the atrial pore is especially sensitive to touch, and 

 a stimulation of this region not only results often in forward loco- 

 motion, but also in a wave of contraction that passes anteriorly from 

 the atrial opening over perhaps half the length of the thin ventral 

 atrial wall. 



As amphioxus is so easily stimulated by gross mechanical disturb- 

 ances, it is not surprising to find that it will respond to such delicate 

 mechanical stimuli as sound waves. If a glass vessel that contains 

 resting amphioxus partly buried in the sand is gently tapped on the 

 side, the animals, as Rice ('80, p. 8) long ago observed, usually with- 

 draw temporarily below the sand, or at least move their cirri in a way 

 that resembles winking. That this is not due to the vibration of par- 

 ticles of sand against their bodies is seen from the fact that at least 

 the reaction of the cirri can be called forth from animals that are rest- 

 ing on a bed of cotton wool in a glass vessel of sea water when the 

 walls of the vessel are tapped. Another common form of response to 

 sound vibrations, often seen under the conditions just mentioned, is a 

 wave-like contraction of the atrial membrane. This membrane in fact 

 is so placed that it may be especially open to stimulation by sound 

 waves, for it is suspended between the atrial cavity and the outer space, 

 both of which are filled with sea water. 



It is very probable that all these reactions to sound depend upon the 

 stimulation of some part of the tactile mechanism, for in the first place 

 amphioxus has no special organ that can serve it as an ear (Stieda, 



