270 THE VITAL FUNCTIONS. 



opening between the segments of the body : they 

 each lead into a respiratory vesicle, situated be- 

 tween the integument and the intestine.* The 

 Leech has sixteen minute orifices of this kind on 

 each side of the body, opening internally into the 

 same number of oval cells, which are respiratory 

 cavities ; the water passing both in and out by the 

 same orifices. -{" 



The Aphrodila aculeata has thirty-two orifices on 

 each side, placed in rows, opening into the abdo- 

 minal cavity, and admitting the water, which is 

 afterwards received into numerous pouches, con- 

 taining caecal processes of the intestine ; so that the 

 nutriment is aerated almost as soon as it is pre- 

 pared by the digestive organs.f 



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 respira- 

 tion consist of long, narrow filaments, in the form 

 of a fringe ; and the blood-vessels belonging to the 

 respiratory system are extensively distributed over 

 the whole surface of these filaments. Organs of this 

 description are denominated Branchia, or Gills; 

 and the arteries which bring the blood to them are 



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

 pages 53 and 148 of his work aheady quoted. 



+ 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 venge cavse of the cephalopoda. 



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



