298 THE VITAL FUNCTIONS. 



vesicle, situated between the integument and the 

 intestine.* The Leech has sixteen minute ori- 

 fices of this kind on each side of the body, open- 

 ing internally into the same number of oval cells, 

 which are respiratory cavities ; the water passing 

 both in and out by the same orifices. ^ 



The Aphrodita aculeata has thirty-two orifices 

 on each side, placed in rows, opening into the 

 abdominal cavity, and admitting the water, which 

 is afterwards received into numerous pouches, 

 containing csecal processes of the intestine ; so 

 that the nutriment is aerated almost as soon as 

 it is prepared by the digestive organs.^ 



In all the higher classes of aquatic animals, 

 where the circulation is carried on by means 

 of a muscular heart, and where the whole of 

 the blood is subjected, during its circuit, to the 

 action of the aerated water, the immediate organs 

 of respiration consist of long, narrow filaments, 

 in the form of a fringe ; and the blood-vessels 



* A minute description of these organs is given by Morren, in 

 pages 53 and 148 of his work already quoted. 



t The blood, after being aerated in these cells, is conveyed 

 into the large lateral vessels, by means of canals, which pass 

 transversely, forming loops, situated between the cseca of the 

 stomach. These loops are studded with an immense number of 

 small rounded bodies of a glandular appearance, resembling those 

 which are appended to the vense cava? of the cephalopoda. 



X Home, Philos. Trans, for 1815, p. 259. 



