GENERAL STRUCTURE OF THE EARTHWORM 131 



fourteenth, and fifteenth somites there 

 are different openings of the reproductive 

 system, while several enlarged somites 

 from the twenty-eighth to the thirty- 

 seventh, forming what is called the clitel- 

 lum are also associated with reproduction. 

 The mouth end of the earthworm is thus 

 differentiated from the remainder of the 

 worm. We can hardly speak of it as the 

 "head" end for there is no head nor tail, 

 but we speak of this type of differentia- 

 tion as antero-posterior differentiation or 

 anterior and posterior ends. In higher 

 types of animals this type of differentia- 

 tion leads to very definite head formation 

 and centralization of the nervous system, 

 while the posterior end always bears the 

 vent or anus (Fig. 52). 



Dorso-ventral Differentiation. The worm 

 always crawls on one surface. If turned 

 over on its "back" it objects vigorously 

 and quickly resumes its normal position. 

 The surface on which it crawls also ap- 

 pears different from the other; it is more 

 flattened, many papilla-like whitish glands 

 are present especially in the anterior part, 

 and the various external openings (mouth, 

 anus, reproductive, excretory) are found 

 here. Furthermore peculiar bristle-like 

 seta, which can be felt by gently drawing 

 the worm between the fingers, are found 

 on this surface. There is, therefore, a 

 fairly well-marked differentiation between 



FIG. 52. Enlarged diagram of the anterior and posterior parts of the earthworm 

 as seen from the ventral side, an, Anus; c, clitellum; g.p., glandular swellings on 

 the twenty-sixth somite; m, mouth; o.d., external openings of the oviducts; PS, 

 prostomium; s, setae; s.r., openings of the seminal receptacles; s.d., external 

 openings of the sperm ducts; 1-40, numbers of the somites beginning behind the 

 prostomium. (From Sedgwick and Wilson.) 



