148 ANNELIDA [CH. 



been mentioned above that the funnel-shaped ciliated openings 

 of the nephridia open into the coelom, so that the fluid of this 

 cavity can pass out of the body not only by the dorsal pores but by 

 the tubular nephridia. This fluid has suspended in it numerous 

 amoebocytes (v. p. 141), and these corpuscles act as scavengers, 

 taking up into themselves any foreign bodies, such as bacteria, 

 which have made their way into the coeloin, and breaking them up. 



The yellow cells (7, Fig. 63), which surround the gut and 

 form the inner wall of the coelom, are also actively engaged in 

 extracting nitrogenous waste from the endoderm cells and the blood- 

 vessels which pass near them. When the excreta have accumulated 

 to a certain extent in a yellow cell it dies, and its remains fall 

 out into the coelomic fluid, where they are eaten by the amoebocytes. 

 These latter then escape by the dorsal pores since the funnel of the 

 nephridium is too small to admit the amoebocytes it serves merely 

 as a flushing apparatus, since its cilia draw in water from the coelom 

 which is swept down the tube and carries the excreta into the 

 terminal bladder whence they are from time to time expelled. 



In the yellow cells we see evidence that the coelomic wall takes 

 part in the process of removing excreta from the blood, and as we 

 ascend in the scale of animal life we shall find that the primitive 

 ectodermic nephridia are more and more replaced by structures 

 derived from the coelomic wall. 



The earthworm, although it lives in earth, has a clean, glistening 

 look, and this is partly due to the fact that the coelomic fluid 

 is poured out from the dorsal pores (11, Fig. 63) and keeps the 

 skin moist and lubricated. This fluid is also antiseptic in its 

 action, and thus its presence prevents foreign organisms, such as 

 bacteria, which swarm in the mould in which the worm lives, from 

 settling upon the skin and growing there. Numerous glandular 

 cells belonging to the ectoderm also pour forth a secretion through 

 minute pores in the cuticle. 



If we cut open an earthworm by a median dorsal incision and 

 attentively examine the upper surface of the pharynx 

 System. 6 we sna ^ nn( l a ^ its anterior end, tucked away between 

 it and the skin, two little whitish knobs lying close 

 to one another. These are the cerebral or supra-pharyngeal 

 ganglia (1, Fig. 60; 2, Fig. 62). At their outer ends the supra- 

 pharyngeal ganglia pass into two cords (3, Fig. 62). If we now 

 cut away the pharynx and remove the alimentary canal we can 

 trace these two cords towards the ventral middle line where they 

 unite and form the first ventral ganglion (4, Fig. 62): from 

 this a long white cord the ventral nerve-cordruns back to 



