Metazoa L umbricus. 24 1 



The purificatory system. For the removal of the pro- 

 ducts of decomposition two kinds of purificatory organs 

 are necessary, viz. those which have for their duty the 

 removal of gaseous products, and those whose function it 

 is to get rid of the nitrogenous waste. We shall first of alt 

 consider the former. 



The respiratory system and respiration. No special 

 organ exists in the earthworm for getting rid of the gaseous 

 excreta, unless the haemal fluid be looked upon as the 

 agent in the process. Respiration is always a double 

 process, and consists of internal respiration, a gaseous 

 interchange between the tissues and the blood on the 

 one hand, and external respiration, a gaseous interchange 

 between the blood and the atmosphere on the other. 

 Blood may in fact be defined from this point of view as the 

 medium by which oxygen is conveyed to the tissues from 

 the air, and by which carbonic acid is transferred from the 

 tissues to the exterior. The haemal fluid is very plentifully 

 distributed to all parts of the body, and thus serves as an 

 agent for the collection of gaseous waste, whilst at the same 

 time it gives up to the tissues oxygen obtained from the 

 external air. Again, the distribution of the fluid in the body- 

 walls admits of its absorbing oxygen from the atmosphere, 

 whilst at the same time it facilitates the excretion of carbonic 

 acid. If this be the case then the two blood fluids of the 

 earthworm would together correspond to the blood of the 

 higher animals the one being respiratory, the other nutri- 

 tive. True blood performs both functions, an obvious 

 economy when the animal type becomes more complicated, 

 as it enables two operations to be performed by one agent, 

 and avoids the necessity of a double pumping mechanism. 



The renal system and excretion. There is no doubt 

 whatever as to the organs for the purification of the body 

 from the nitrogenous excreta. The renal system consists of 

 a large number of coiled tubes complicated in structure and 

 distributed in pairs, one pair in each somite of the body 



R 



