102 PLANARIA ARETHUSA. 



some which is black as jet. The mouth of 

 the proboscis appears at c in Fig. 15. of 

 the planaria nigra, viewed by a magnifier. 



The perplexities occasioned from dis- 

 coloration, were illustrated by various ex- 

 amples, which shew the difficulty of guard- 

 ing against error. On the twenty-eighth 

 of January 1813, I took a number of plana- 

 riae indiscriminately from the spring at the 

 Well-house Tower. None of the arethu- 

 sae could be detected among them ; but 

 several bearing an exact resemblance ex- 

 cept in colour. All the latter were brown 

 of different shades ; the head, margin, and 

 site of the proboscis clear ; and in some, 

 pinnate ramifications, filled with a bright 

 red substance, appeared. The exemption 

 of the head, margin, and proboscis, from 

 the colour, led me to conjecture that it was 

 accidental, for no perceptible vessels tra- 

 verse these parts ; but in every respect the 

 deception was complete. On the fifth of 

 February, so great an alteration had ensued 

 ag to verify my anticipations ; yet it was 



