l54 EARTHWORMS. PHYLUM ANNELIDA 



by the presence of the typhlosole. The contractions which cause 

 the passage of the food are alternately caused through the nerves 

 to the pharynx and inhibited through the plexuses in the septa. 

 From the anus faeces and undigested soil are passed out as the 

 familiar worm casts (in Allolobophora) or in the burrows below 

 the surface. The function of the oesophageal glands is probably 

 the excreting of the calcareous matter which is very plentiful 

 in the dead leaves of which the food is largely composed. Possibly 

 their secretion is also of importance in removing carbon dioxide 

 in the form of calcium carbonate. They are characteristic only 

 of those species of worm which are large and Hve in a relatively 

 dry environment. 



EXCRETION 



Besides the yellow cells of the intestine, the earthworm 

 has excretory organs which, like those of vertebrates, consist 

 of tubes with walls that are glandular and excretory and richly 

 supplied with blood vessels ; but the tubes, instead of being 

 collected into compact kidneys, are distributed along the body, 

 one pair to each segment, except the first three and the last which 

 have none. Each tube or nephridium is thrown into loops, bound 

 together by connective tissue containing blood vessels. The 

 nephridium (Figs. 113, 114) begins as a flattened, kidney-shaped 

 funnel or nephridiostome hanging from the front side of a septum 

 near the nerve cord. The nephridiostome has an overhanging 

 lip which consists of a large crescentic central cell with a row 

 of marginal cells around it. This lip is ciliated. The lower lip is 

 not ciliated. From the funnel theie leads a narrow tube, ciliated 

 on its sides. This passes through the septum to the main part of 

 the nephridium, which lies behind the septum, in the coelom 

 of the next somite, opening to the exterior by the nephridiopore 

 in that somite. The narrow part of the tube is long and winding 

 and loses its ciha in places. It is followed by a wider, short, 

 brown region, cihated throughout, this by a still wider tube 

 which is not ciliated, and finally a short, very wide, muscular 

 tube leads to the nephridiopore. The whole tube, except the 

 muscular region, is formed of hollow cells shaped like drain-pipes 

 and lying end to end. The middle part of the nephridium stores 

 excretory granules probably throughout hfe, and a fluid is also 

 driven to the exterior, being liberated by the opening of the 

 nephridiopore once every three days. The chief excretory con- 



