PHYLUM ANNELIDA 



2 3 I 





! 



. 



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 w r hich 

 was so changed that " after 

 thirty years (1871) a horse 



COUld gallop over the Com- FlG . 161. Section through the upper 

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



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 cind ers buried by worm castings. 



shoes" (Fig. 161). 



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. 



D, subsoil not disturbed by the earth- 

 worms. (From Schmeil.) 



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. 



