THE EARTHWORM 283 



The Earthworm 



Occurrence and Distribution. — The common earthworm, Lum- 

 hricus terrestris, is selected as representative of the Annulata 

 because it is a familiar animal and because it illustrates in a satis- 

 factory manner the biological principles of structure and function 

 which it is desired to present in connection with this phylum. It is, 

 however, a more specialized type of annulate worm than some of 

 its relatives that inhabit the ocean. Earthworms of various genera 

 and species are found in most parts of the world, even on isolated 

 oceanic islands, wherever the ground is sufficiently moist and where 

 the temperature is not too low. In localities where they are 

 abundant, the distribution is determined by the nature of the 

 soil and its moisture. Heavy clay soils and thin soils in which 

 the underlying rock is near the surface are not favorable, while 

 rich loamy and sand}'' soils are the most advantageous. 



External Features. — Before describing the burrowing and 

 other general activities of the earthworm, it is desiraljle to review 

 the more important external features (Fig. 137 A). The elongated 

 cylindrical body is bluntly pointed anteriorly and somewhat 

 compressed dorso-ventrally in its posterior region. The 77iouth 

 is at the anterior end, surrounded by the first segment, or peri- 

 stomium, and just ventral to a small protuberance, the prostomium. 

 The anus is a vertical slit at the posterior extremity, in the ter- 

 minal or anal segment. Dorsal and ventral surfaces may be 

 recognized by the difference in color, the dorsal being darker as 

 in most animals, and also by a slight flattening of the ventral 

 region which appears throughout the greater portion of the 

 length (cf. Fig. 140). The body is conspicuously marked off into 

 a series of segments, or metameres, of which there are about one 

 hundred and seventy-five in a full-grown specimen of Lumbricus 

 terrestris. There is a swollen area, the clitellum, which, at the 

 time of egg laying, secretes the egg capsules in which the young 

 develop, and which is usually located in the region between 

 segments twenty-nine and forty-one. Other special skin glands 

 appear as lighter areas on certain of the anterior segments. The 

 openings of the vasa deferentia, or male genital ducts, are found 

 on the fifteenth segment as apertures surrounded by conspicuous 

 lips. The openings of the oviducts are much smaller and located 

 on the fourteenth segment. Between segments nine and ten and 



