93 



a kind of exudation ; while, in the mean time, 

 numerous air-tubes, beginning from the surface 

 of the body, are ramified over this single dorsal 

 vessel, and serve to keep its contents in the re- 

 quisite state of purity. In other animals, as we 

 have seen, the blood is carried at intervals to the 

 air, or other medium by which it is purified : 

 in most insects, on the contrary, the air is car- 

 ried to the blood, and how conducive this must 

 be to the end just alluded to, must be sufficiently 

 obvious. The dorsal vessel, with the air-tubes 

 distributed upon it, may be distinctly seen with a 

 microscope, and even observed to pulsate, in the 

 bee, the silk worm, and numerous other perfect 

 insects ; while in many of these animals in the 

 state of larvae, there is, according to Carus, 

 a circulation of the blood more corresponding 

 to that of other animals, the blood-vessels be- 

 coming obliterated only in proportion as they 

 would have been inconvenient. And, according- 

 ly, in many of the Apterous insects there is the 

 usual complement of arteries and veins, and even 

 a proper heart placed between the two. In the 

 cray-fish, for example, the heart is situated im- 

 mediately below the dorsal shield, being distinctly 

 seen to pulsate when this is removed, and sends 

 off several arteries both forwards and backwards ; 

 and in the soldier's crab the heart has a similar 

 situation, and gives off its vessels in a similar 

 manner. 



Of the vertebral animals, the heart is single in 

 fishes arid reptiles, as it is in the two preceding 



