620 SERGIUS MORGULIS 



that the worm may move in a given direction of its own accord, 

 so to speak, i.e., regardless of extraneous forces. Doubtless much 

 of what I shall say has already been stated by Jennings ('06), 

 in his excellent paper on the earthworm's movements, but the fact 

 is not yet fully appreciated that the earthworn has an internal 

 mechanism of precise orientation, whereby several modes of its 

 behavior, hitherto unnoticed may now be explained. The 

 worm tends to move in a straight line and, even in spite of ob- 

 stacles, it will follow in the chosen course with obstinate persistency. 

 If the head of a worm moving in a definite direction is pushed to 

 the right or left side, it invariably returns to its former position, 

 and the worm continues its straight course. If an obstacle, such 

 as a lead block, is so placed across the worm's path that the worm, 

 persisting in its course, must impinge upon the block at a right 

 angle, the animal will retract its front portion and often back 

 up some distance as soon as it comes in contact with the block. 

 A few seconds later, however, it resumes the straight forward 

 course only to bump once more against the resistance. This per- 

 formance of intermittent bumping may last for some time, and 

 yet the worm will persist in its course without swerving to the 

 right or left side. But when the block is so situated that the angle 

 of impact of the moving worm is less than 90° the animal either 

 creeps alongside the block, or turning its head in searching move- 

 ments until the obstacle is ultimately avoided, starts on a new 

 course. 



The tendency of the worm to move and to maintain itself per- 

 tinaciously in a definite direction has led me to inquire if there is 

 present an internal mechanism of orientation which determines 

 the course of the animal's movements at any particular moment. 

 A simple apparatus was constructed for this purpose consisting 

 of two plates of slate: a large plate firmly fixed to the support, and 

 a small sector so pivoted to the first that it could be moved easily 

 in a horizontal plane, as will be seen in the diagram (fig. 1) A 

 pair of guards, also of slate, were at times attachedAm either side 

 of the pivot in order to prevent the worm from swerving off at 

 the moment of passing from one plate to the other, but this part 

 of the apparatus was not essential and could generally be dis- 



