PLANARIA ARETHUSA. 99 



is protruded far without it, a question will 

 arise, how is so remarkable a change ac- 

 complished ? In a state of rest, the pro- 

 boscis occupies the place denoted by an el- 

 liptical spot externally visible ; and then it 

 is a short hollow cylinder, of greater dia- 

 meter at the mouth. Its extreme dilatibi- 

 lity is effected by reducing the sides of the 

 tube in extension, which reduction is per- 

 haps chiefly manifested towards the ex- 

 tremity, where the substance is more accu- 

 mulated ; and we must observe, that while, 

 from facility of expression, it is called a 

 hollow cylinder, the whole interior circum- 

 ference is probably in contact. This is a 

 wise provision of nature. The structure of 

 the proboscis intimately corresponds with 

 the general structure of the body ; it is 

 susceptible of the same expansion and con- 

 traction; and the incessant change of shape 

 in the one is attended by a similar altera- 

 tion in the other. If, by a dexterous opera- 

 tion, the extended organ be severed from 

 the animal while feeding, an experiment 



