CHLAMIDOMONAS. 99 



them for any time ; Pritchard says, that " when- 

 ever it is attempted to retain them in large 

 quantities, the second day will generally exhibit 

 a thick mass of dead ones at the bottom of the 

 vessel. When a few only remain alive, if the water 

 be poured away, and the creatures removed into 

 a vessel of clear water, they will live for weeks.' 3 

 They are most abundant in the spring of the year. 

 A green matter is often observed on ponds 

 and other places, which at a first glance might be 

 taken for a minute weed in great quantities, but 

 if examined with care, it will be found en- 

 tirely formed of the animalcule belonging to the 

 genus CJdamidomonas pulvisculus, so named from 

 its forming a dust-like stratum on the surface. 

 They are often in vast quantities, and when they 



