THE WORMS 55 



ANNELIDS OK SEGMENTED WORMS 



55. The earthworms and their relatives. Leaving the 

 groups of the parasitic animals, which have been driven from 

 the field of active existence and in many ways are degraded 

 by such a mode of life, we pass on to the higher free-living 

 worms, where brilliant colors, peculiar habits, or remarkable 

 adaptations render them peculiarly interesting. In consid- 

 ering first their general organization, we may use the earth- 



FIG. 33. Earthworm (Lumbrlcus terrestris). m, mouth ; c, girdle or clitellum. 



worm (Fig. 33) (sometimes called angle-worm or fish-worm) 

 as a type because of its almost universal distribution. 



The body is cylindrical, shows well-marked dorsal and 

 ventral surfaces, and, as in all of the annelids, is jointed, 

 each joint being known as a segment. Anteriorly it tapers 

 to a point, and the head region bearing the mouth is ill- 

 defined, unlike many sea forms, yet serves admirably for 

 tunneling the soil in which all earthworms live. In this 

 process the animal is also aided by bristles or setce which 

 project from the body wall of almo'st every segment and 

 may be stuck into the earth to afford a foothold. 



56. Foodjmd digestive system. The earthworms are 

 nocturnal animals, seldom coming to the surface during the 

 day except when forced to do so by the filling of their tun- 

 nels with water or when pursued by enemies. At night 

 they usually emerge partially, keeping the posterior end of 

 the body within the burrow, and thus they scour the sur- 

 rounding areas for food, which they appear, in some cases 

 at least, to locate by a feeble sense of smell. They also 

 frequently extend their habitations, and in so doing swallow 

 enormous quantities of earth from which they digest out 

 any nutritive substances, leaving the indigestible matter in 

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