110 PLANARIA ARETHIFSA. 



■fished up a number of the arethusae now 

 under discussion, from a spring well about 

 half a mile distant. All were entire ; no 

 separated fragments appeared ; nor had 

 I for long observed any divisions among 

 those kept in my apartment. On the 

 whole, it rather seems to me, that the 

 circumstance of remov^al from the place of 

 their natural origin, renders these animals 

 more subject to spontaneous division of the 

 body ; and in general it succeeds soon af- 

 terwards. There are exceptions, it is 

 true, which are not easily susceptible of 

 explanation ; and conjecture alone can 

 ascribe them to age, sex if there is sex, 

 or the distinction of it in the arethusae, 

 or to their preservation from any kind of 

 constraint or suffering. Those of certain 

 waters are, besides, apparently less liable 

 to it than the planariae of others, which 

 may infer a difference in the species diffi- 

 cult to be recognised by observers. But 

 with the planaria? felinse such partition is 

 incessant in every different size, and in all 



