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by gills we may take the cray-fish, in which the 

 gills are situated on the sides of the animal at the 

 base of the feet, and separated from the other in- 

 ternal organs by a horny plate, consisting of dis- 

 tinct pieces, something like the parts of the chest 

 in the higher tribes of animals : it is furnished 

 also with two other horny pieces attached to the 

 jaws, by means of which the water, already viti- 

 ated by the contact of the gills, is expelled from 

 their surface. The bee again presents an ex- 

 ample of insects which breathe by respiratory 

 bags, of which it has two, opening on the surface 

 of the body by two holes stigmata, as they are 

 called and giving rise to several branched tubes. 

 On the other hand, the respiratory apparatus of 

 the grub of this insect, as indeed of most others, 

 is exclusively tubular; and these tubes have a 

 very different distribution from that which they 

 present in the perfect insect. The same is the 

 case with the caterpillar of the silk worm ; but 

 even in the perfect animal, in this instance, the 

 respiratory apparatus is tubular alone. Of these 

 tubes one large one runs along each side of the 

 body, and gives off, opposite to each of the nu- 

 merous openings upon the surface, two sets of 

 branches, one to the lower part of the body, and 

 the other to the upper, in such a manner that 

 the former branches go chiefly to the muscles 

 moving the feet, and the latter to the dorsal 

 blood-vessel, and to the several entrails, which 

 in insects are always situated near their back. 

 The stigmata, or orifices of the respiratory tubes, 



