46 PLANARIA FELINA. 



assemble in a quiescent state, on the side 

 next the hght. Some others, which natu- 

 rahsts have classed in the genus, repose on 

 the side opposite the light; — so decided 

 are the characters which regulate the dis^ 

 position of animals. 



This planaria, like the rest of its genus, 

 is powerfully excited to motion by the pre- 

 sence of light. Then it traverses the sides 

 of its glass with a lively gliding progres- 

 sion, or swims supine at the surface of the 

 water ; but neither it, nor any which I have 

 seen, except the flexilis, ventures to com- 

 mit itself entirely to that element. If 

 dropping from the surface to the bottom, 

 which is a characteristic of all this and the 

 next division of the genus, it seems to ex- 

 ercise a faculty belonging to the caterpillar, 

 of spinning a silken thread, visible only 

 when so much aggregated as to interrupt 

 the rays of light. Its use is evidently to 

 check the rapidity of descent ; and a com- 

 plete view of its effect may be obtained by 

 including a plant of Veronica, crowded with 



