100 



The last subject which is to occupy us at pre-. 

 sent, is the respiratory organs, or those by which 

 the blood, received into its vessels from the ali- 

 mentary canal, is, during its subsequent circula- 

 tion through the body, kept in a state of requi- 

 site purity. This is in all cases effected by bring- 

 ing it, at intervals, into contiguity either with 

 atmospheric air alone, or with water containing- 

 this air diffused through it ; when, such is the 

 mutual action of the blood and the air upon each 

 other, that the former is purified, and passes in 

 general from a dingy purple to a bright scarlet 

 colour, while the latter is, in the same degree, 

 rendered impure, and after a time becomes ina- 

 dequate to support either respiration or combus- 

 tion. In the simplest forms of animals, most of 

 the zoophytes, for example, respiration seems to 

 be what is called by St. Hilaire, interstitial, that 

 is to say, to go on in every point of their bodies 

 equally ; the best glasses having failed in detect- 

 ing any specific apparatus for the purpose, al- 

 though it is known that they do respire, by the 

 changes which the air, left for a certain time in 

 contact with their bodies, is found to have un- 

 dergone. In some of these, however, as the 

 branched polype and sea-feather, as well as some 

 testaceous and mollusous animals, as the sea- 

 urchin, star-fish, sea-anemone, &c., the arms, 

 tentacula or animal blossoms, which they respec- 

 tively put forth, are, in all probability, like the 

 leaves of plants, more particularly adapted to re- 

 spiration ; although no specific pores for this 



