DIFFERENTIATION AND SYMMETRY. 



119 



one end, obtusely rounded and flattened at 

 the other. As a rule, the pointed end 

 moves forwards in locomotion, and the 

 mouth opens near it. For these and other 

 reasons the pointed end might be called the 

 head-end, and the other the tail-end. But 

 the worm has really neither head nor tail, 

 and hence the two ends may better be dis- 

 tinguished as the fore-end and the kinder- 

 end, or still better as anterior and posteri or, 

 And in scientific language the fact that the 

 worm has anterior and posterior ends which, 

 differ from each other is stated by saying 

 that it shows antero-posterior differentia- 

 tion. This simple fact acquires great im- 

 portance in the light of comparative zoolo- 

 gy ; for it may be shown that the antero- 

 posterior differentiation of the earthworm, 

 insignificant as it seems, is only the begin- 

 ning of a series of important modifications 

 extending upwards through more and more <40 

 complex stages to culminate in man himself. 

 Dorso-ventral Differentiation. In living 

 or well-preserved specimens, the body is not 

 perfectly cylindrical, but is somewhat flat- 

 tened, particularly near the posterior end, 

 and has a slightly prismatic four-sided form. 

 One of the flattened sides, slightly darker 

 in color than the other, is habitually turned 

 upwards, and is therefore called the back, 

 the opposite or lower side, commonly turned 

 downwards, being the belly. For the sake 

 of accuracy, however, biologists are wont to 

 speak of the dorsal aspect (back) and ventral 

 aspect (belly) of the body ; and the fact that 



FIG. 72. Enlarged view of the anterior and posterior parts 

 of the body of an earthworm as seen from the ventral as- 

 pect, an, anus; c, clitellum; g.p, glandular prominences on 

 the 26th somite; i, mouth; o.d, external openings of the 

 oviducts; p.s, prostomium; s, setae; s.r, openings of the 

 seminal receptacles; s.d, external openings of the sperm- 

 ducts. (Slightly enlarged.) 



