136 ZOOLOGY FOR MEDICAL STUDENTS chap. 



nephridium is really embedded in the septum, the substance of which 

 it causes to bulge freely into the neighbouring cavities — the funnel into 

 the compartment in front of the septum, the main part of the nephridium 

 into the compartment behind. The portion of nephridium next the 

 external opening consists of a tubular bladder or reservoir with muscular 

 walls — a favourite haunt of small parasitic nematode worms. The 

 remainder, apart from the funnel, consists of a protoplasmic strand con- 

 taining scattered nuclei, through which passes backwards and forwards, 

 folded twice upon itself, a tubular cavity, varying in diameter in different 

 regions and bearing over great parts of its inner surface actively moving 

 cilia. 



The function of the nephridium is a renal or excretory one and it 

 performs this function in two ways, (i) The wall of the nephridium 

 is richly supplied with blood by numerous vessels, and as the blood 

 circulates through these, the protoplasm of the nephridial wall extracts 

 from it the nitrogenous waste products of metabolism and passes 

 them on into the tubular cavity. (2) The cilia of the nephrostome cause 

 a slow current of the watery coelomic fluid to set outwards through 

 the funnel and down the cavity of the tube. This current serves to carry 

 away excess of coelomic fluid and small particles of disintegrated yellow 

 cells which may be floating about in it. The outgoing stream also 

 incidentally assists the first-mentioned function, inasmuch as it serves 

 to flush out the excretory substances passed into the cavity of the 

 nephridium by the activity of its wall. 



The other communications between the coelome and the exterior 

 are much simpler than those afforded by the nephridia, being in the 

 form of direct openings in the mid-dorsal line known as the dorsal pores 

 (Fig. 62, dp). The dorsal pore is situated close to the anterior boundary 

 of the somite, immediately behind the septum, so that its external opening 

 lies in the groove which demarcates its somite from that immediately 

 in front of it. A dorsal pore occurs in each somite except the first 

 ten. Light is thrown on the function of the dorsal pores if a small 

 drop of some irritating substance is placed on the skin of a live worm. 

 Drops of milky coelomic fluid are seen to exude from the dorsal pores 

 in the neighbourhood so as to wash away the irritant. If the irritating 

 substance be a little drop of a culture of some irritating bacterium the 

 amoebocytes in the exuded drop of coelomic fluid attack the bacteria 

 and ingest them. No doubt the dorsal pores play an important part 

 in the protection of the skin against the attacks of bacteria or other 

 injurious organisms by allowing free egress for coelomic fluid containing 

 the defensive amoebocytes. 



