Chapter 20 



A SEGMENTED WORM— EARTHWORM 

 (PHYLUM ANNELIDA) 



The common earthworm is Lumbricus terrestris (lum'brikus te -res'- 

 tris) (Gr. lumbrikus, earthworm; L. terra, earth). Its body is elongated, 

 soft, and segmented. It burrows in the soil by forcing the soil through 

 its alimentary tract and passing this soil on the surface as ''castings." A 

 conspicuous saddlelike clitellum (kli -tel' um) (L. clitellae, pack-saddle) 

 is present (segments XXXI to XXXVII) . A true body cavity or coelom 

 (se' lom) (Gr. koilos, hollow) exists internally and communicates with 

 the exterior by means of dorsal pores in the mid-dorsal line at the an- 

 terior edge of each segment from VIII to the posterior end of the body. 

 Membranous, internal septa (L. septum, partition) separate adjacent 

 segments (Figs. 185 to 190). 



Integument. — -The thin, noncellular, transparent cuticle of ectodermal 

 origin is iridescent because of its striations to refract light to produce 

 various colors. The cuticle contains pores of glands located in the epi- 

 dermis (hypodermis) just beneath. Four pairs of chitinous, bristlelike 

 setae (se' ti) (L. seta, bristle) are located in seta sacs in each segment 

 (Fig. 187). 



Motion and Locomotion. — An outer layer of circular muscles and an 

 inner layer of longitudinal muscles in the body wall aid in locomotion. 

 Four pairs of bristlelike setae per segment are moved by protractor (pro- 

 trak' tor) (Gr. pro, forth; tractus, to draw) and retractor muscles 

 (re -trak' tor) (L. retrahere, to draw back) at their bases. Each seta 

 is set in a seta sac. The setae are set at certain angles to provide friction 

 or to reduce it as desired. 



Ingestion and Digestion. — The foods of earthworms consist of dead 

 plant and animal materials and soils rich in organic substances. Living 

 materials are rarely molested. A fleshy prostomium (pro -sto' mi um) 

 (Gr. pro, before; stoma, mouth) projects over the mouth at the anterior 

 end. The mouth leads into a buccal pouch which connects with a thick, 



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