134 



ORGANS AND ORGAN SYSTEMS 



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 reproduc- 

 tion. 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 differ- 

 entiation as antero-posterior differentia- 

 tion, or anterior and posterior ends. In 

 higher types of animals this type of dif- 

 ferentiation leads to very definite head 

 formation and centralization of the nerv- 

 ous system, while the posterior end al- 

 ways bears the vent or anus (Fig. 52). 



Dorso-ventral D ijfe rentiation . T h e 

 worm always crawls on one surface. If 

 turned over on its "back," it objects vig- 

 orously and quickly resumes its normal 

 position. The surface on which it crawls 

 also appears 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. Further- 

 more peculiar bristle-like setae, 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 the 



FIG. 52. Enlarged diagram of the anterior and posterior parts of the earth- 

 worm as seen from the ventral side, an, Anus; c, clitellum; g.p.. glandular swell- 

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

 ps, prostomium; s, setae; 1 -?.;*., 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.) 



