00 ^ PLANARIA ARETHUSA. 



disposed to condescend on the last, and to 

 lay down, that the leading species is always 

 grey in its original abode, and darker as it 

 is larger. Very fine specimens of both 

 inhabit a spring at the north-west base, of 

 the rock surmounted by the castle of Edin- 

 burgh, close to its issue from a ruin called 

 the Well-house Tower. 



I have also, but rarely, found a planaria 

 almost exactly of the same description as 

 what is denominated the leading species, 

 except in being of deeper grey, the body 

 somewhat of a more slender, equal, . and 

 vermicular form, the tentacula less promi- 

 nent, and the head broader at the extre- 

 mity. The frequency of the animal has 

 not been sufficient, nor the distinctions so 

 evident, as to enable me to prove whether 

 it is a variety, or should be disjoined from 

 the species. Possibly such discrepancies 

 may originate in sparing aliment, the ab- 

 sence of light, or the temperature of the at- 

 mosphere. It is inconceivable on how many 

 conditions the real external aspect of these 



