ANTERO-POSTERIOR DIFFERENIIA TION. 



43 



GENERAL MORPHOLOGY. 



Attention will first be directed to certain features of the 

 BODY seemingly of little importance, but really full of meaning 

 when compared with like features in other 

 animals higher or lower in the scale of 

 organization. 



Antero- posterior Differentiation. The 

 body (Fig. 21) has an elongated cylindrical 

 form, tapering to a blunt point at one end, 

 obtusely rounded and flattened at the other. 

 As a rule, the pointed end moves for- 

 wards 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 distin- 

 guished as tliefore end and the hinder end, 

 or still better as anterior and posterior. 

 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 differ- 

 entiation. This simple fact acquires great 

 importance in the light of comparative 

 biology; for it may be shown that the 

 antero-posterior differentiation of the earth- 

 worm, insignificant as it seems, is only the 

 begining of a series of important modifica- 

 tions extending upwards through more and 

 more complex stages to culminate in man 

 himself. 



FIG. 21. Enlarged view of the anterior and posterior 

 parts of the body of an earthworm as seen from the 

 ventral aspect, an, anus ; c, clitellum ; (/.p., glandular 

 prominences on the 36th somite ; in, mouth ; o.d, exter- 

 nal openings of the oviducts ; p.*., prostomium ; s, setae ; 

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

 openings of the sperm-ducts. The form of the body 

 varies greatly in life according to the state of expan- 

 sion. The specimen here shown is from an alcoholic 

 preparation. (Slightly enlarged.) 



