PLANARIA FLEXILIS. 7 



singly, advanced in a straight line from 

 the respective clusters. Microscopical ob- 

 servations are attended with extreme dif- 

 ficulty, owing to the incessant motion of 

 the creatures, and the necessity for their 

 being kept in water ; and they contract so 

 much when exposed to cold, that render- 

 ing them torpid is of little utility. 



It may here be remarked, that the exa- 

 mination of pale and pellucid animals with 

 lenses, is greatly facilitated by their inter- 

 position between a dark ground and the mag- 

 nifier, as also that it should uniformly take 

 place by day. All the artificial light which 

 the combination of combustible substances 

 can produce, is infinitely inferior to the 

 slightest gleam of sunshine, or even what 

 is transmitted from a clouded sky. The 

 higher the magnifying power, the greater 

 the consequent obscuration of the object, 

 which proves how dependent our know- 

 ledge of its structure is on the degree of 

 illumination that it receives. 



Whether these specks be truly eyes, or 



