290 



BIOLOGICAL LECTURES. 



rolling up into the form of a ball is the safest course of behavior 

 and the one generally adopted. 



2. Forced by Attack. — The same behavior will almost inva- 

 riably follow when any species of Clepsine is closely pursued 

 and finds itself unable to fix itself by either end, as when a 

 spatula is repeatedly thrust under it in such a way as to break 

 its hold and defeat its efforts to regain a footing. 



3. Induced by Gorging Blood. — The provocations to such 

 behavior thus far considered have all been such as might, and 

 probably do, cause more or less alarm. It is important to note, 

 however, that the instinct may manifest itself frequently under 

 conditions that seem to exclude the influence of fear. I have 

 often seen these leeches fold themselves into balls at the end 

 of a good meal, and so roll to the edge of the shell of their 

 host, and fall to the bottom. This mode of concluding a quiet 

 repast, with no assignable cause for alarm, and with every 

 reason for satisfaction and contentment, except for the desire 

 to get out of light into darkness, under cover of a stone or 

 some other object, will hardly pass as feigning; and cataplexy 

 and the tropisms are equally out of question. We could not 

 assume, for example, that Clepsine is positively heliotropic 

 when hungry, and negatively heliotropic after feeding. Shade 

 is preferred at all times in both conditions. If hungry, Clep- 

 sine leaves the shade, not because it prefers light, but because 

 it prefers its host more than it prefers shade. If the host is 

 not found it will again return to a shady retreat, if one is to 

 be found, however hungry it may be. The rolling up cannot 

 be attributed to light, as the animal takes the extended posi- 

 tion when at rest, even if compelled to remain in the light. 

 What, then, shall we conclude } 



4. Origin and Utility. — Observation and inference may be 

 stated as follows : 



I . The act of rolling up into a passive ball may be performed 

 (a) under compulsion, as when it is the last resort in self- 

 defence ; (b) under a milder provocation, as one of three courses 

 of behavior, as when the resting place is turned up to light, 

 and the choice is offered between remaining quiet in place, 

 creeping away at leisure, or rolling into a ball and dropping to 



