ANNELIDA OR ANNULATA 119 



Physiology, Anatomy, and Locomotion.— The Chaetopoda are 

 made up of similar segments or metameres. At the sides are borne 

 chaetae or setae^ which are in some forms attached to muscular 

 processes called parapodia. The well-developed body cavity or 

 coelom contains the alimentary canal, the vascular system and 

 respiratory branchiae. The excretory organs, nephridia^ are ar- 

 ranged in pairs in each segment except the first three and the last. 

 Sexes are separate in some forms while others are hermaphroditic. 

 The nervous system consists of paired dorsal cerebral ganglia and a 

 ventral chain of ganglia with lateral nerves. The larval form is 

 called a trochophore . The Naidae are fresh water forms which bud 

 asexually. The Hirudinea have 2^o\xX five external annnlations to 

 each internal segment and have an extremely distensible crop for 

 the storage of blood. They are hermaphroditic. They swim with 

 great rapidity. * The aquatic forms have an undulatory movement, 

 while the land forms, like the ordinary earthworm, contract the 

 circular muscles of the body, thus elongating the segments, and then 

 having fixed the setae in the ground, by contraction of the longitu- 

 dinal muscles, direct the movement of the worm either anteriorly or 

 posteriorly. 



Regeneration. — Earthworms will regenerate a head or a tail, 

 sometimes forming a tail in place of the head, and starving to death. 

 Grafting and fusion to form two-headed or two-tailed individuals 

 have been successful. 



Fossil Relatives. — The Chaetopoda are found as fossils from the 

 Cambrian to the present, while the Hirudinea are unknown as fossils. 



Ancestry and Relationship to Other Phyla. — There seems to be a 

 well-defined connecting link between the leeches and the allies of the 

 earthworm, since we find that the leech Acanthobdella has setae and a 

 well-developed coelom. 



There seems to be a wide gap between the Platyhelminthes and 

 the Annulata, since both metamerism and a different nervous system 

 characterize the latter. Some would hold that the hair worm, 

 Gordius^ is a close relative of the Annulata. On account of the 

 common Trochophore larva, some have linked the Trochelminthes 

 with the Annulata. (See page 125.) 



Economic Importance of the Earthworm 



Positive. — {a) Charles Darwin estimated that there were 50,000 

 worms in one acre of ground and cited a stony hill covered with earth 



