PLANARIA ARETHUSA. 91 



animals depends ; the mere passing of a 

 cloud before the sun will produce so ma- 

 terial and sudden an effect, that a total 

 change in shape and motion, consequently 

 in colour, ensues. Nor is it difficult to 

 comprehend that it must be so, by reflect- 

 ing on the extreme dilatability and con- 

 tractility with which every portion of a 

 planaria is endowed ; that in progression 

 its length is at least quadruple as much as 

 when at rest ; that its tentacula can disap- 

 pear at will, and that the whole proportions 

 of its body can be changed. Want or abun- 

 dance of food will occasion analogous altera- 

 tions in colour, size, and figure ; new ap- 

 proximation or separation of parts arises, 

 and the animal is hardly seen twice identi- 

 cally the same in every thing. Repeated 

 observation is therefore the sole means of 

 appreciating the relation of all these com- 

 binations. 



In searching for the permanent charac- 

 teristics which might distinguish the pla- 

 nariae now described, I subjected two of 



