238 Structiire and Economy of the Anmilate Antmals, 



sible kind was all that was required, and is all that has 

 been given. It is well known that the blood of animals, when 

 deprived of all communion with atmospheric air, stays in its 

 course, and ceases to perform its functions. Now, by the 

 simple system of circulation above described, it is evident that 

 the blood could not be brought, with any regularity, to one 

 particular spot, there to communicate with and receive air 

 sufficient to renew and maintain its vital power. It becomes, 

 therefore, a matter of necessity, that, as the blood does not 

 go to the air to receive revivification, the air must be intro- 

 duced to the blood for effecting that purpose. The most 

 simple, the most direct, way of accomplishing this was by 

 piercing the sides of the animal at regular intervals through- 

 out its length, and inserting tubes through which the atmo- 

 spheric air should pass with perfect freedom into the system, 

 and mingle with the blood in any quantity that nature might 

 demand. This is the very plan that we find has been adopted. 



To this, the breathing of the marine annulates, already 

 noticed, may be cited as an exception ; it may be said that 

 their blood is cold, and yet they breathe by gills. This is 

 true ; but it is also true that their residence in the depths of 

 the ocean precludes the possibility of their using spiracula ; 

 we therefore find the system of their circulation raised, to 

 cooperate with their improved respiration. It was quite 

 needless that their blood should, farther, be warmed; for we 

 find that even in fishes the blood is perfectly cold, for the 

 same reason as in insects, because their bodies are (when 

 young) too small to resist the temperature of the surrounding 

 medium. 



There are numerous aquatic annulates which belong to the 

 hexapod or true insects, which are provided with a system of 

 respiratory organs precisely similar to that of the terrestrial 

 insects ; these have the power of enveloping, with a bladder 

 of atmospheric air, those parts of the body in which their 

 spiracles are situated ; thus not only protecting the spiracles 

 from receiving any injury from the ingress of the water, but 

 keeping up the necessary supply of air to the blood. The 

 bladder of air is renewable at pleasure by rising to the sur- 

 face, * 



( To be continued.) 



* Do not the sacs, within which trachean ^rachnides have been found pa- 

 rasitic upon the aquatic i>ytlscus marginalis (see p. 161.) serve the same 

 purpose as this " bladder of atmospheric air" serves to the aquatic hexapod 

 insects ? and, farther, all the cysts, with which a variety of animals pa- 

 rasitic upon aquatic animals are found covered ? See VI. 94., for a notice 

 of some parasites found encysted. — J.D. 



