PHYLUM ANNELIDA 



23! 



The faeces of these worms are the little heaps of black earth, 



called "castings" which strew the ground, being especially 



noticeable early in the morning. Darwin estimated that more 



than eighteen tons of earthy 



castings may be carried to the 



surface in a single year on one 



acre of ground, and in twenty 



years a layer three inches 



thick would be transferred 



from the subsoil to the sur- 



face. By this means objects 



are covered up in the course 



of a few years. Darwin 



speaks of a stony field which 



was so changed that " after 



thirty years (1871) a horse 





O 



Could gallop over the COm- FIG. 161. Section through the upper 



pact turf from one end of the stratum of a field showing the work of 



\ earthworms. .4 and B, arable soil 



held to the Other, and not thrown up by earthworms. C, marl 



Strike a single Stone with its and cinders buried by worm castings. 

 . D, subsoil not disturbed oy the earth- 



Shoes (tig. l6l). worms. (From Schmeil.) 



The continuous honeycomb- 



ing of the soil by earthworms makes the land more porous and 

 insures the better penetration of air and moisture. The 

 thorough working over of the surface layers of earth also helps 

 to make the soil more fertile. 



2. CLASSIFICATION OF ANNELIDS 



Definition. - - Annelids are segmented worms, the body 

 consisting of a linear series of more or less similar parts. Many 

 of the internal organs are segmentally arranged, notably the 

 blood-vessels, excretory organs, and nervous system. A large 

 perivisceral ccelom is usually present, and in some cases a tro- 

 chophore stage (Fig. 162) appears in development. Setae are 

 characteristics of the majority. 



