EXCRETORY SYSTEM OF EARTHWORM 



251 



work on the pharynx ; the dorsal and the supra-neural are 

 linked together in the region of the gullet by five or six 

 pairs of contractile vessels or " hearts." 



Respiration is efl[^ected by the distribution of blood on 

 the general surface of the skin. 



Excretory system. — There is a pair of nephridia in each 

 segment except the first four. Each opens internally into 



Fig. 135. — Transverse section of earthworm. 



.4., Cuticle; B., epidermis; CM., circular muscles; L.M., longitudinal 

 muscles ; D., a seta ; C, coelom ; Y.C., yellow cells ; F., typhlosole ; 

 V.V., supra-neural blood vessel ; S.V., sub-neural vessel ; DA' ., dorsal 

 vessel; P., peritoneum ; £., cavity of gut ; G., endodermiclining of gut ; 

 N., part of a nephridium ; R., opening of a nephridium ; /., the nerve- 

 cord ; /., a nerve given off ; K., giant fibres in the nerve-cord. 



the segment in front of that on which its other end opens 

 to the exterior. They remove little particles from the body 

 cavity, and get fluid waste products from the associated 

 blood vessels. Nephridia occur in many animals, in most 

 young Vertebrates as well as among Invertebrates, but thev 

 are never seen more clearly than in the earthworm. When 

 a nephridium is carefully removed, along with a part of 

 the septum through which it passes, and examined under 



