CHAPTER XXXIX 



EARTHWORM 



A TYPE OF THE PHYLUM ANNELIDA 



Earthworms are very generally distributed, being absent only from 

 regions where the soil is nearly pure sand and deficient in humus or 

 from mountain regions where the soil is scanty and also poor. Very 

 heavy soils with much clay in them are not favorable, and earthworms 

 are unable to live in soils that are strongly acid. During the daytime 

 they remain in their burrows in the ground, but at night they may come 

 out and move about on the surface. A species common in Europe and 

 America, Lumhricus terrestris Linnaeus, may serve as a type. 



269. External Characteristics. — The most prominent characteristic 

 of the earthworm is the division of its body into metameres to the number 

 of 175 in fully mature individuals, not including a half metamere at the 

 anterior end known as a prostomium (Fig. 144). The metameres, which 

 may be numbered XXXI or XXXII to XXXVII are, in mature worms, 

 swollen and whitish, the skin containing glands which take part in repro- 

 duction. These metameres form a region known as the clitellum. 



The surface of the worm is covered by a thin transparent cuticula 

 marked by fine striae which produce iridescence. The cuticula is pierced 

 by bristle-like setae, which are present in all but the anterior two or 

 three metameres and the last. These setae are very small chitinous rods 

 similar in form to an old-style s (/) and serve as organs of locomotion. 

 There are four pairs to a metamere, one pair on each side just below the 

 middle and the other on each side midway between the first pair and the 

 ventral median line. 



The mouth is a crescent-shaped opening on the anterior side of the 

 first metamere, overhung by the -prostomium. The anal opening is a 

 slit on the posterior side of the last metamere. Minute dorsal pores 

 located in the mid-dorsal line at the anterior margin of each metamere 

 behind the eighth or ninth are openings from the coelom directly to the 

 outside. Other openings on the surface will be referred to in connection 

 with the organs of elimination and reproduction. 



270. Internal Structure. — The body wall of the earthworm surrounds 

 (Fig. 145) a cavity which is called the coelom; this is separated into com- 

 partments by cross partitions, or septa, which mark the boundaries of 

 the metameres. These partitions are more or less incomplete or even 

 absent between certain metameres at the anterior end of the body. From 



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