84 PLANARIA NIGRA. 



active; all as the universal shade of the body- 

 darkens. Yet exposure to the atmosphere, 

 unmixed with another fluid, is inevitable 

 destruction to the planaria. If, chancing to 

 wander beyond the confines of its native 

 element, its endeavours to return be inter- 

 rupted, it contracts and grows distorted, a 

 kind of gluten issues from the whole body, 

 and death speedily ensues. Should it be 

 hastily removed, it may recover, and then 

 the gluten comes off like a sheath, or inte- 

 gument, investing the body ; but the tran- 

 sition must be immediate. Whether this 

 substance be provided by nature, to avert 

 the baneful effects of unguarded exposure, 

 or exudes in consequence of the sufferings 

 of the animal, is uncertain. 



In the natural state, the planaria proba- 

 bly survives the cold of congelation ; yet it 

 perished during the course of an observation, 

 where the thermometer stood at 26^, and 

 where it was imbedded in a solid mass of 

 ice. Perhaps the change was too sud- 

 den ; for numerous societies dwell in shal- 



