LECTURE XIL 
217 
malcules appears, at first view, almost a hopeless 
labour; since exclusive of the vast variety of 
species, (of which, in all probability, only a small 
part has yet been observed,) many of them have 
a power of changing their shape at pleasure ; so 
as to appear widely different at particular times 
from what they did the moment before ; and others, 
though their form is constant, are apt to vary in 
colour; by which means some deception or ob- 
scurity may arise, and an tmcertainty in determin- 
ing the species. Much, however, has been done; 
a great many species of Animalcules have been 
perfectly well described, and are perfectly well 
known to microscopical observers, since they 
possess characters too clear and plain to admit of 
any doubt of their species, whenever they happen 
to appear. 
As examples of this curious and interesting 
race of animals I shall particularize a few of the 
most remarkable kinds, and such as are well figured 
in the works of Naturalists. 
Among these the genus called Vorticella is one 
of the principal. Its character is, that the mouth 
or opening is surrounded by numerous short 
feelers, forming a kind of fringe round the head. 
