AND ITS BIOLOGICAL SIGNIFICANCE. 



333 



axis of the spiral, — the animal continues to swerve toward the dorsodextral side till 

 its general direction is completely changed. Figure J represents such a reaction in 



Diurella tigris Miiller. 



Now it is evident that if in 



this turning as a result of a 



stimulus the animal strikes its 



delicate head against anything, 



it will always be the dorsodextral angle which is 



thus struck, and we see again why the tooth 



should be at this point. 



Sometimes the animal runs forward into a 



small angle, where it cannot directly turn, as, for 



example, between the surface film of water and the 



bottom of a watch-glass. In this case the animal 



turns, as usual, toward the dorsodextral side, but 



as a result it may merely bump its head against 



the bottom. It nevertheless perseveres, trying to turn 



in the same direction, while it revolves also on its long 



axis. Thus the head will be dragged and bumped along 



the surface, until the dorsodextral side (through the 



revolution on the long axis) becomes directed toward 



the free water. Then of course the animal swerves off 



in this direction. 



No one who has seen this peculiar performance 

 (which is not at all uncommon) can remain in doubt as 

 to what is the significance of having the tooth or teeth 

 at the dorsodextral side of the anterior end of the lorica. 

 These teeth take all the "bumping," while the animal is 

 turning, in place of its falling on the delicate corona. 

 The teeth are placed just where they will serve to pro- 

 tect the head when the anterior end comes in contact 

 with anything. No teeth are found in the correspond- 

 ing position on the left side, because they would serve 

 no purpose there. 



Finally, the twisting of the foot and toes so that 

 they can bend only to the right finds its explanation along the same line. The 

 entire animal is constructed on the plan of turning to the right, both during its 



Fig. J. — Reaction of Diurella tigris 

 Miiller to a stimulus, while freely 

 swimming, a; represents an object 

 against which the animal strikes. 

 It then turns (at 3) farther than 

 usual toward the side which bears 

 the tooth and ridge, and continues 

 the spiral course. 



