88 PLANARIA ARETHUSA. 



accustomed food, or the ravages of disease ; 

 and it is not unlikely that some fatal at- 

 tack, such as that which resolves the pla- 

 naria panniculata into a flocculent sub- 

 stance, has eradicated many of the kindred 

 species. 



In three pure springs, and also at the 

 bursting of a very faint discharge from the 

 ground, I have obtained a planaria in certain 

 respects analogous, which I never observed 

 in stagnant waters. Above, it is generally 

 of a leaden grey, deeper in proportion to 

 its size ; the under part is pale, with a line 

 of different hue, proceeding from the origin 

 of the head to within a third of the oppo- 

 site extremity, which denotes the site of a 

 large intestine. Its figure is more slender 

 and delicate than that of any of the other 

 fresh water species here described ; flattish 

 like them when in a state of abstinence, and 

 forming the segment of a cylinder if feed- 

 ing copiously. It is about half an inch long, 

 and the twelfth part of an inch in breadth ; 



