PHYSIOLOGY OF THE INVERTEBRATA. 



257 



(2) The Oligochaia. — The renal system of Lumhrims consists 

 of a large number of coiled tubes (Fig. 47) distributed in pairs, 

 one pair in each somite of the body. Each tube or segmental 

 organ (nephridium) consists of three distinct parts— (a) A 

 much-convoluted thin portion, terminating in a funnel-shaped 

 opening ; (h) a thick-walled glandular portion ; (c) a thick 



Cuticle. -^ 

 Epidermis. 



Circular mus- -=■ 

 cular layer. 



Longitudinal 

 muscular 

 layer. 



Outer loop of 

 nephridium. 



E.xternal 

 opening of 

 nephridium. 



Dorsal vessel. 



» Middle loopof 

 nephridium. 



Typhosole. 



^ Hepatic cells, 

 so-called 

 " liver." 



Inner loop of 

 nephridium. 



^ Epithelium of 

 intestine. 



Ventral v. 



Ccelom. 



Internal open- 

 ing of neph. 



Vent, nerve 

 cord. 

 » Subneural 

 vessel. 



Fig. 47.— Nephkioium of Lumbricus. 



muscular portion (the outer loop), which opens externally by 

 an aperture near the ventral side of the body. The nephri- 

 dium as a whole lies on the posterior side of the septum, but 

 the funnel-shaped aperture opens on the anterior surface ; 

 that is to say, into the cavity of the segment in front of that 

 in which the main body of the nephridium lies. This is the 

 case in every segment containing these organs. Tlie septa, 

 or mesenteries dividing the body into segments, are richly 



