FORMATION OF ANIMAL AGGREGATIONS 39 



water, where they may collect in great aggregations unless scattered 

 by waves or by tidal currents. 



These larvae swim in a long spiral path orienting quite accurately 

 to light. The orientation, Mast says (191 1), is not entirely accurate 

 but is subject to frequent muscular turnings which result in re- 

 orientations. The general course is toward the light, as shown by 

 the diagram (Fig. i). The following account of the details of this 

 reaction is taken from Mast's description (1911), since he has been 

 consistently critical of interpreting any animal reaction as approach- 

 ing automatonism. 



"If the direction of the rays of light is changed after the larvae are 

 oriented, they all appear to turn directly toward the source of light 

 in its new position without preliminary trial movements." Ordinari- 

 ly, these larvae swim so rapidly that the exact details of their path 

 are hard to follow. When caught under a sloping cover slip so that 

 they can no longer swim spirally, if the larvae are caught lying on 

 one side no definite movement is seen except a slight forward mo- 

 tion; in those lying on either dorsal or ventral surface, the anterior 

 end is seen to move constantly from side to side with a slight jerky 

 motion, a movement undoubtedly due to muscular contractions. If 

 light is thrown on such an organism at right angles, the lateral move- 

 ment toward the illuminated side is at once increased, and the larva 

 turns in that direction. "By using two sources of light so situated that 

 the rays cross at right angles in the region where the specimen is lo- 

 cated, and then alternately intercepting the light from each of the two 

 sources, it can be seen clearly that the larva, by muscular move- 

 ment, turns the anterior end toward the source of light directly. 

 There is no trial reaction in this process. It is an asymmetrical re- 

 sponse to an asymmetrical stimulation. The movement of these an- 

 nelid larvae appear little more voluntary than the precise movement 

 of algal swarm spores." 



Galvanotropic reactions frequently produce aggregations in a 

 diagrammatic fashion. Thus Paramecium, a protozoan well known 

 to react usually by a reflex type of behavior which suggests Jennings' 

 designation of a "trial and error" reaction, exhibits a forced-move- 

 ment type of behavior under the influence of a continuous electric 



