IV LUMBRICUS 13s 



filled with a watery coelomic fluid in which creep about numerous 

 amoebocytes {am). These amoebocytes constitute the mobile defence 

 force alluded to on p. 130, they are physiologically phagocytes, i.e. 

 their function is to devour and destroy noxious foreign particles such 

 as bacteria which may have penetrated into the body of the worm. 

 The coelomic cavities are bounded all round by the coelomic epithelium 

 already mentioned. For the most part this consists of very thin flat 

 cells, but on the outer surface of the intestine^ and also over the surface 

 of some of the blood-vessels, these are modified as yellow cells (Fig. 

 63, y.c) — compactly shaped cells whose cytoplasm deposits within itself 

 granules of yellowish-brown excretory material which gives the cell its 

 characteristic colour. As the amount of this excretory substance increases 



Fig. 66. 



A nephridium of the Earthworm (after Maziarski). n.o, External opening ; «s, nephrostome ; 



s, septum. 



the cell-body may become so clogged up that life is no longer possible, 

 the cell becomes moribund, it dies and disintegrates, and its remains are 

 partially disposed of by the phagocytes, partly conveyed to the exterior 

 by other means presently to be mentioned. 



A typical coelomic compartment is not completely closed but com- 

 municates with the outer world by three openings. Two of these are 

 paired and form the kidney tubules or renal tubules — known technically 

 as the nephridia (Fig. 67, n). 



Of these nephridia there is a pair in each somite except the first three 

 and the last. The nephridium is a coiled slender tube (Fig. 66). At 

 its inner end it communicates with the coelomic cavity by a richly ciliated 

 coelomic funnel or nephrostome (Fig. 66, ns) while at its outer it opens 

 (n.o) on the surface of the body somewhat ventrally and laterally. The 



