S. S. MAXWELL 23 



rection of the stimulating object. This rotation effect would be in- 

 creased by the new position of the pectoral fins. The pectoral on 

 the stimulated side is elevated but its posterior margin is raised less 

 than its anterior or is even depressed; the pectoral of the other side 

 makes a movement which is just the converse. These fins would 

 then also have a screw effect tending to the same direction of rotation 

 as the dorsals, namely, ventral side toward the stimulating object. 

 The reaction is clearly tropic and in the positive sense. 



It will be seen that the total effect of a weak stimulus is to turn the 

 ventral side of the animal, as well as to swerve the course, in the 

 direction of the stimulating object. This accords well with what one 

 sees on watching the dogfish swimming about in a small aquarium. 

 They are often seen going round and round, keeping near the walls, 

 with the body tilted to one side so that the mouth and belly are turned 

 somewhat toward the wall. This is just the position which would 

 be produced by the above reactions, if, on making the turn at a corner, 

 the edge of the snout came slightly in contact with the wall. Some- 

 times I have been able to see such contacts actually occurring, but 

 the asymmetrical position was often assumed when the wall was not 

 touched. In this case it might be that the increased pressure or 

 resistance of the water when the fish was moving near the wall could 

 act as a stimulus. Indeed I found that a spurt of water from a pipette 

 could be used instead of a finger stroke as a weak stimulus. 



Since in the dogfish the mouth is far back on the ventral surface 

 of the head it is not unreasonable to suppose that the positive stereo- 

 tropic reaction assists in the capture of food; the response to a con- 

 tact stimulus would tend at once to bring the mouth into position 

 to seize the stimulating object. 



Strong Stimuli. — The experiments described above show that the 

 reaction to a strong stimulus is almost exactly the reverse of the 

 reaction to a weak stimulus. The dorsal fins and the tail are flexed to 

 the side away from the contact. The pectoral fin on the stimulated 

 side is elevated, its posterior margin still more than its anterior, 

 the pectoral on the opposite side is depressed, the posterior margin 

 more than the anterior. The whole arrangement of the fins is that 

 of a screw whose effect in the water would be to rotate the body around 

 its longitudinal axis in such a way as to turn the back to the stimulat- 



