ADDITIONS TO CHAP. III. 
OF THE CHANGES INDUCED ON THE AIR BY THF, 
RESPIRATION OF ZOOPHYTES, WORMS, MOLLUSC A* 
INSECTS, FISHLS, AND REPTILES, 
533. IN our former chapter on the respiration of 
the lower animals, we followed nearly the classifica- 
tion of the Linnaean school ; but the farther we 
have advanced in our inquiries, the more have we 
perceived its imperfections. When treating simply of 
the changes induced on the air by the living functions 
of animals, we did not deem it necessary to enter 
minutely into the peculiarities of the animal struc- 
ture, since it was our object, at that time, rather to 
ascertain the general facts with respect to the air, 
than to describe the organs by which respiration is 
performed, or the effects which it produces in the 
system. In the prosecution of this last branch of 
our subject, we have been obliged to enter on a more 
minute survey of the animal system ; and it became, 
therefore, an object of importance to adopt an ar- 
rangement that should lead us through the chief va- 
