250 
tions ; but has replaced the two genera, Cancer 
and Monoculus, among the apterous insects. It is 
tjie arrangement of this author which we propose to 
follow in the remaining part of this work, because it 
appears to us to be the most natural, precise, and 
comprehensive ; and, being founded entirely on ana- 
tomical distinctions, it is best suited to display that 
progressive variation in the structure and functions 
of animals, which it will hereafter be our business to 
describe and explain. 
535. Conformably to these views, we shall consi- 
der the lowest and most simple animals as forming 
a distinct class, under the appellation of Zoophytes. 
In many individuals of this class, the necessary con- 
currence of water and heat to the commencement of 
living action has been already exemplified (48. et 
seq.) ; and the same agents are essentially necessary 
to the development of the ova, from which other 
animals of this class proceed. Spallanzani removed 
the liquid from a number of the eggs of animalcules, 
so as to leave them perfectly dry, and they remained 
unchanged for ten days ; but on putting them into 
their native liquid, they were soon hatched *. So, 
likewise, he found, that, although great cold did not 
destroy the living faculty, in many animalcular ova, 
yet it suspended their evolution, which was always 
promoted by a due degree of heat. 
536. With respect to the operation of air, the 
same author has remarked, that he never found any 
animalcule to be produced in vegetable infusions 
* Tracts on Aiiim. and Veg. p, 66. 
