102 



parous snail, on the contrary, which is an aqua- 

 tic animal, and in which this respiratory bag 

 would be useless, is furnished instead of it with re* 

 gular gills. Lastly, in some animals of this class, 

 the respiratory apparatus consists of a series of 

 small blind tubes, running along the body, on the 

 surface of which they open, and apparently 

 adapted equally well to the respiration of water 

 and air ; since they are found, among the aqua- 

 tic animals, in the leech, and, among the terres- 

 trial, in the earth-worm. These tubes are tra- 

 versed, as I have already remarked when speak- 

 ing of the circulation of the blood in the earth- 

 worm, by one or two veins which return the 

 blood to the chief artery ; but as the course of 

 the other vein is quite independent of these tubes, 

 it is easy to understand why the circulation is 

 not immediately stopped in these animals, by 

 a stoppage of respiration. In the snail and 

 cuttle, on the contrary, any impediment to the 

 action of the respiratory organs, immediately 

 stops the circulation of their blood, the heart re- 

 ceiving its blood exclusively from these organs, 

 preparatory to sending it through the body in 

 general. 



The respiratory apparatus of insects is very 

 similar to that of worms, in so far at least as it 

 consists of either gills, respiratory bags or tubes j 

 although the two latter are in general much 

 more complicated in their structure, and diffused 

 more or less throughout the whole body of the 

 animal. As an example of an insect breathing 



