PLANARIA ARETHUSA. 107 



their native abode; for they gradually, 

 though slowly, decline and wasti away. 

 Perhaps this may partly he ascribed to the 

 inconsiderable quantity of water devoted to 

 their use, compared with what they enjoy 

 in their natural state ; thence, rendering 

 them more liable to be affected by external 

 causes ; or it may arise from an exhaustion 

 of those principles adapted for the preser- 

 vation of life. The want of mud and other 

 substances into which they might retreat 

 from the impressions of the light and air, 

 may also operate their destruction ; but its 

 chief source centers in some noxious prin- 

 ciple contracted by water. If remaining a 

 considerable time unchanged, the planaria3 

 decrease more rapidly, they become lan- 

 guid, scarcely moving either by the influ- 

 ence of light or heat, and at last adhere 

 entirely to the side of the containing ves- 

 sel, where they perish. Relative to this 

 subject, I remember that when engaged in 

 experiments on these animals in the city 

 of Edinburgh, though thriving well, and 



