ANIMAL BEHAVIOR. 289 



b. Rolling into a Ball. 



Clepsine has another and entirely different method of keeping 

 quiet. The behavior bears striking analogy to that which has 

 been described as "feigning death" in some insects. The 

 animal rolls itself up (head first and ventral side innermost) 

 into a hard ball, outwardly passive, and free to roll or fall 

 whithersoever gravity and currents in the water may direct it. 

 The ball will bear considerable pressure and rough handling 

 without unfolding or exhibiting any marked movements. Left 

 in quiet for a few seconds, the animal slowly unrolls itself and 

 creeps off. This instinct has many advantages for a slow- 

 moving creature like Clepsine, as will presently be seen. 



I . Provoked by Exposure. — The ball-like attitude is assumed 

 under various circumstances. If a stone or board with Clepsine 

 attached to its under surface be quietly turned upside down, 

 thus bringing the leech from shade or darkness into light and 

 exposure, it may sometimes maintain its position of rest un- 

 changed, only hugging the stone a little more closely and not 

 moving until all is quiet. More generally, however, it rolls 

 itself up, and by the time the stone is turned, or before, it falls 

 to the bottom, where it can unfold and escape without danger 

 of discovery. If, by chance, the animal has eggs, it will not 

 desert them to escape in this way. As soon as the eggs hatch 

 and the young become attached to the ventral side of the 

 parent, the latter may roll itself up with its brood inside, 

 fall to the bottom as before, and thus escape with all its 

 progeny. 



This species, then, has two quite distinct and peculiar ways 

 of keeping quiet and thus avoiding its enemies. If the animal 

 has no eggs, or if it has young, it may adopt either mode of 

 escape, while if it has eggs it has no choice but to remain 

 quiet over them. In the species here considered, the eggs are 

 held together in a thin, gelatinous sheet, secreted at the time 

 of ovipositing, and of a size and form to be entirely covered 

 by the expanded body of the parent. In some species of Clep- 

 sine the eggs are laid in thin membrane-like sacs, which are 

 fastened to the under side of the parent, and in this case the 



