140 THE INVERTEBRATA 



the amount in solution is used up. In air, on the other hand, free 

 carbon dioxide will accumulate, but there is a large supply of oxygen. 

 Consequently aquatic respiration will sooner be affected by changes 

 in the pressure of oxygen in the medium, aerial respiration by changes 

 in the pressure of carbon dioxide. In foul waters, however, free 

 carbon dioxide may be present in such quantities as to be an important 

 factor. In such waters, and in the habitat of many internal parasites, 

 free oxygen may be practically absent. Many animals which live in 

 such circumstances obtain energy by an anaerobic process, the 

 complex molecules of carbohydrates being decomposed to form 

 simpler ones without the intervention of free oxygen. To this end the 

 animals in question lay up in their tissues large quantities of the 

 starch-like substance glycogen of which carbohydrate stores in 

 animals are usually composed. Thirty per cent, of the dry weight of 

 an Ascaris, and nearly half that of a tapeworm, consist of this sub- 

 stance. The glycogen is converted into glucose (dextrose) and then 

 decomposed, according to the following equations: 



(i) (C,n,,0,)n-^nU,0 = nC,U,,0, 



(ii) C,U,,0, = 2C,Ufi, 



(Glucose) (Lactic acid) 



This process of course yields considerably less energy than would be 

 obtained by total oxidation. Apparently, at least in many animals, 

 it cannot go on indefinitely unless the lactic acid be removed. This 

 may happen either by the acid being discharged into the surrounding 

 fluid and swept away by movements of the latter (which would occur, 

 for instance, in the host's intestine), or by an access of oxygen with 

 which some of the lactic acid is oxidised so as to give energy for 

 building the rest back into glycogen. In the latter case the process 

 becomes ultimately aerobic. It is probable, indeed, that even in 

 aerobic animals the process by which energy is liberated is at first 

 anaerobic, but that this phase is quickly followed by one in which, 

 by the use of oxygen, a part of the product is destroyed and the rest 

 built back, so that the process as a whole appears aerobic. Thus 

 anaerobic animals differ from those that are aerobic only in the length 

 of time for which the anaerobic process goes on. 



Organs of excretion are even more various, in kind and in origin, 

 than those of respiration. If the removal of carbon dioxide from the 

 body be disregarded, there are two processes to be considered here — • 

 the excretion of water and that of solids. In the lower aquatic animals, 

 whose surface is in various degrees permeable to water, the removal 

 of the latter from the body is, as we have seen, a matter of very great 

 importance: it was probably the original function of the nephridial 

 system and is an essential part of that of excretory coelomoducts. But 



