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. t 



The Aphrodita acideata has thirty-two orifices 

 on each side, placed in rows, opening into one 

 large respiratory sac, which is situated immedi- 

 ately under the muscles of the back, but sepa- 

 rated by a membrane from the abdominal cavity. 

 Projecting into this sac, are seen several mem- 

 branous vesicles, fifteen in number on each 

 side, which have no external opening, but which 

 receive, on the inner part, the ends of certain 

 tubes, or caeca, sent off' from the intestinal canal ; 

 so that the nutriment is aerated almost as soon 

 as it is prepared by the digestive organs.J 



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 



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

 pages 53 and 148 of his work aheady 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 convey the venrR cavse of the cephalopoda. 



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



