PLANARIA ARETHUSA. 93 



cent, and by a constriction in the middle, 

 occasionally seemed almost divided asun- 

 der. Their position appears in Fig. 13. 

 which represents one of the pale animals 

 somewhat magnified. An instance occur- 

 red, where, by a monstrous conformation of 

 the head, a planaria had a third eye situa- 

 ted to the right of the other two. 



The organ of vision, if it be truly such, 

 is not the only one whose site or opera- 

 tion is obscure in planariae. I had studied 

 the history of several species of this tribe 

 during successive years, without being able 

 to discover the precise manner in which 

 they fed, though repeated experiments 

 were made for that purpose. But I cannot 

 deny my having been partly misled by the 

 authority of nomenclators, who, founding 

 their conclusions on superficial and preci- 

 pitate remarks, too often forget the nature 

 of an animal, in their anxiety to give it a 

 name. Nomenclature, so useful when ap- 

 propriately employed, ought to rest on a 

 general combination of the structure and 



