AQUATIC RESPIRATION. 213 



respiratory cavity, before it can be swallowed, and received 

 into the stomach. 



In several of the ftnnelida, also, we find internal organs 

 of respiration. The Lumbricus terrestris* or common 

 earth-worm, has a single row of apertures, about 120 in num- 

 ber, placed along the back, and opening between the seg- 

 ments of the body: they each lead into a respiratory vesicle, 

 situated between 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.t 



The JUphrodita aculeata has thirty-two orifices on each 

 side, placed in rows, opening into one large respiratory sac, 

 which is situated immediately under the muscles of the back, 

 but separated by a membrane from the abdominal cavity. 

 Projecting into this sac, are seen several membranous vesi- 

 cles, 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.! 



In all the higher classes of aquatic animals, where the cir- 

 culation 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 belonging to the respiratory 

 system are extensively distributed over the whole surface of 



* A minute description of these organs is given by Morren, in pages 53 

 and 148 of his work, already 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, si- 

 tuated between the cxca of the stomach. These loops are studded with an 

 immense number of small rounded bodies of a glandular appearance, resem- 

 bling those which convey the venae cavae of the cephalopoda. 



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



