272 GENERAL BIOLOGY 



From what has been said above it will be seen that there is in reality 

 no true circulation in the earthworm. 



RESPIRATION 



The earthworm needs oxygen just as do all animals ; but, as it has 

 no lungs it obtains its oxygen through its moist outer membrane. There 

 are many capillaries lying immediately beneath the cuticle, thus pre- 

 senting a great expanse of blood area which is somewhat similar to the 

 many capillaries in the lungs of higher forms. The oxygen here com- 

 bines with haemoglobin. The blood gets to these capillaries through 

 the vessels supplying the body wall and is then returned to the dorsal 

 trunk by way of the sub-neural trunk and the intestinal connectives. 



As the nervous system must co-ordinate every movement of the 

 body, it requires an excellent blood-supply, which is furnished the bet- 

 ter in the earthworm by the sub-neural trunk lying very close to the 

 ventral nerve cord. The nervous system is thus continually supplied 

 with fresh nourishment. 



THE EXCRETORY SYSTEM 



Most of the excretory matter is carried outside the body by a num- 

 ber of coiled tubes called nephridia, a pair of which lies in each somite 

 except the first three and the last. The dorsal pores also serve as ex- 

 cretory organs to a minor extent. 



A clear understanding of the nephridia is important, because such 

 an understanding will serve the student in good stead in his future 

 studies of the excretory organs of vertebrates. This is the better un- 

 derstood when it is known that the excretory organs of all higher forms 

 develop from embryological beginnings quite similar to those of the 

 earthworm. 



Each nephridium (Fig. 168) consists of: 



(1) The funnel or nephrostome ( ), 



(2) The ciliated neck, 



(3) The coiled narrow tube, 



(4) The wide glandular tube, 



(5) The ejaculatory duct opening to the outside. 



"The ciliated neck of the nephrostome passes through the anterior 

 wall of the somite, close to the mid-ventral line. The nephrostome, 

 therefore, lies in the somite anterior to the one containing its own 

 nephridium, so that waste matters of any one somite are expelled to the 

 outside by the nephridium of the next posterior somite. The nephro- 

 stomes, or mouths, of the nephridia are flattened fan-like structures, con- 

 sisting of two flattened lamellae or plates with a narrow slit-like opening 

 between them ; the great cells lining the opening are covered with pow- 

 erful cilia which maintain a constant current toward the tubular part 

 of the nephridium. These tubes are developed in coils which lie in the 



