278 ZOOLOGICAL PHILOSOPHY 



into their alimentary cavity ; and by their incessant expansions have 

 greatly developed this cavity, have induced the radiating form of 

 these animals both internally and externally, and moreover cause 

 the isochronous movements observed in the soft radiarians. 



When nature had estabUshed muscular movement, as she has in 

 the insects and perhaps even a httle before, she had a new means for 

 increasing the movement of their serum or essential fluid ; but on reach- 

 ing the organisation of the crustaceans, this means no longer sufi&ced, 

 and a special system of organs had to be created for accelerating the 

 essential fluid or blood of these animals. It is indeed in the crustaceans 

 that we find for the first time a complete general circulation ; for this 

 function is only rudimentary in the arachnids. 



Every new system of organs acquired is permanently preserved in 

 subsequent organisations ; but nature continues to work towards its 

 gradual perfection. 



The general circulation is thus at first provided only with a heart 

 with one ventricle, and indeed in the annelids even a heart is un- 

 known : it is at first accompanied only by an incomplete respiratory 

 circulation, viz. one in which all the blood does not pass through the 

 organ of respiration before being despatched to the parts. This is 

 the case with animals which have imperfect gills ; but in fishes, where 

 the branchial respiration is perfect, the general circulation is accom- 

 panied by a complete respiratory circulation. 



When nature subsequently created lungs for breathing, as she did 

 in the reptiles, the general circulation was of necessity accompanied 

 only by an incomplete respiratory circulation ; because the new 

 respiratory organ was still too imperfect, and because the general 

 circulation still had a heart with only one ventricle and also because 

 the new fluid breathed is by itself more effectively restorative than 

 water, so that a complete respiration was not needed. But when 

 nature reached that perfection of pulmonary respiration seen in the 

 birds and mammals, the general circulation came to be accompanied 

 by a complete respiratory circulation ; the heart necessarily had two 

 ventricles and two auricles ; and the blood gained its highest velocity ; 

 the high animalisation became capable of raising the animal's internal 

 temperature above that of the environment, and, lastly, the blood 

 became subject to rapid decomposition requiring corresponding 

 restoration. 



The circulation of the essential fluid of a living body is then an 

 organic function peculiar to certain animals : it first becomes complete 

 and general in the crustaceans, and is afterwards found gradually be- 

 coming more perfect in animals of the following classes ; but it would 

 be vain to seek it in the less perfect animals of the anterior classes. 



