PHYLUM ANNELIDA 



231 



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 (187 1) a horse 

 could gallop over the com- 



f^pffK^'*l| 



Fig. 161. — Section through the upper 



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



r . . , , - earthworms. A and B, arable soil 



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



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



v. 11 /!?• \ ^' su bsoil not disturbed by the earth- 



Shoes (lug. 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. 



