348 Dr Grant on the mfluence of Light 



viridis of Muller and Bruguiere), which is perceptible to the 

 naked eye, and has a briglu grass-green colour. This animal- 

 cule is found in the summer season in stagnant pools of fresh 

 water, where it forms a thin green film on the surface. It was 

 observed by Muller in this situation in the fresh water pools 

 of Denmark, and it is found in the same situation in stagnant 

 pools near London. Muller observed that when these animal- 

 cules were placed in a vessel of water, they collected at the 

 margin, and died by the evaporation of the water, leaving a thin 

 green film on the side of the vessel. In the month of August 

 last I observed a light-green film on the south side of a small pool 

 of stagnant fresh water near London ; it covered detached por- 

 tions of the surface and extended over more than twenty square 

 feet. As it did not appear to the naked eye to be produced 

 by the green leaves of any plant, I placed a small portion of 

 the film in water under a pocket microscope and observed that 

 the whole green matter detached itself into separate lively ani- 

 malcules with a tapering bifurcated tail, and corresponding ex- 

 actly with the figures and descriptions of the Cercaria viridis, 

 given by Muller in his Animalc. Infus. (tab. 19, fig. 6 — 13), 

 and by Bruguiere in the Encyc. Meth. (pi. 9, fig. 6 — IB). 

 Lamarck has made a distinct genus, Fitrcocerca, of those Cer- 

 carice of Muller which, like the present species, have a bifur- 

 cated tail. 



The water containing these minute animalcules was placed 

 in a shallow crystal vessel near a window that I might observe 

 their motions and appearance. Under the microscope they ex- 

 hibited a granular or vesicular texture, but presented none of 

 those spots which Muller mistook for eyes in some other spe- 

 cies of Cercarice. After remaining about two hours I observed 

 my green animalcules all accumulated at the surface of the 

 water on one side of the vessel, and nearly left dry on the 

 shallow margin by the rapid evaporation of the water. Think- 

 ing that some slight inclination of the vessel to one side might 

 have caused them thus to accumulate at one part of the mar- 

 gin, I turned that part slowly to the opposite side, added a 

 small supply of water, and agitated slightly with the water the 

 animalcules which had nearly perished by the evaporation. 

 On inspecting them a few hours afterwards I found them all 



