ANIMAL BEHAVIOR. 291 



the bottom ; (c) or, finally, under no special external stimulus, 

 but rather from internal motive, the normal demand for rest 

 and shady seclusion, presumably very strong in Clepsine after 

 gorging itself with the blood of its turtle host. 



2. This mode of taking leave of the turtle, after a full meal, 

 is the easiest, the quickest, and the safest way available. To 

 drop off fully extended, as Clepsine sometimes does, would 

 retard descent and increase the chances of capture by fish. 

 To creep about on the back of the host, waiting for an oppor- 

 tunity to grasp a plant or stone, would be decidedly hazardous, 

 for if it came within the stretch of its host's neck, annihilation 

 would be almost certain, while if lucky enough to keep out of 

 reach of its host, it would still be in danger of the same fate 

 from other turtles. 



3. This behavior is instinctive, since it is performed by the 

 young after the first meal as perfectly as by the adult. 



4. Looking more closely at the nature and origin of this 

 instinct, it will be seen to be quite a natural performance, in 

 keeping with the most fundamental features of the animal's 

 organization, and only a special application of a more general 

 act that is primary and organic as much as tasting, seeing, or 

 sleeping. 



The more general act consists simply in tucking or rolling the 

 head under, as often happens when the animal is resting. The 

 habit may be observed in the young as soon as they are suffi- 

 ciently developed to be capable of bending the tip of the head 

 under. The same act, carried a little further, gives the half- 

 rolled condition, in which only the anterior of the animal is 

 folded, while the posterior portion remains unrolled and attached 

 by the sucker. This attitude is often assumed if the leech is 

 sick or has been injured. It is only a step further to release the 

 sucker and fold it over the part already rolled up, thus com- 

 pleting the part ball to a whole ball, which can move passively 

 more rapidly and safely than is possible by active creeping. 

 From beginning to end we have only one act, in different stages 

 of completion, simply different degrees of one and the same 

 process. 



5. Having the general act to start with, it is easy to see 



