(LOGY KOR MEDICAL STUDENTS CHAP. 



.;11\ embedded in the septum, the substance of which 

 iv into the neighbouring cavities the funnel into 

 ipartment in" front of the septum, the main part of the nephridium 

 Mipartment behind. The portion of nephridium next the 

 Mini: consists of a tubular bladder or reservoir with muscular 

 .\ourite 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, 

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

 and bearini: over i;reat parts of its inner surface actively moving 



iimction 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 

 circulate> through these, the protoplasm of the nephridial wall extracts 

 trom it the nitrogenous waste products of metabolism and passes 

 i int<> 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 

 I' coclomic fluid and small particles of disintegrated yellow 

 elU 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 



.'. <//). The dorsal pore is situated close to the anterior boundary 



"mite, immediately behind the septum, so that its external opening 



. e 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 



1C irritating substance is placed on the skin of a live worm. 



of milky roelomic fluid are seen to exude from the dorsal pores 



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



i!tl- drop of a culture of some irritating bacterium the 



- vto in the exuded drop of coelomic fluid attack the bacteria 



and m-e^t them. No doubt the dorsal pores play an important part 



n ..I the skin against the attacks of bacteria or other 



ma by allowing free egress for coelomic fluid containing 



