200 



TEXTBOOK OF ZOOLOGY 



remains constant, while the number posterior to this varies. This 

 is because growth is accomplished by the addition of segments poste- 

 rior to the clitellum. 



The prostomium is a sort of knoblike lobe at the anterior end, 

 projecting out over the mouth. It is not considered a true meta- 

 mere. The first segment is incomplete due to the opening of the 



Pros'f'omi'u/n 



XXVI- 



xxxrr-i 



xxxvir- 



Openjngf 

 ofoviducf 



^Open!n0 of 

 "^Mpis deferens 



^Sem/'ncfl 

 % groove 



zm 



, C//fe//u/7i 



^Sefae 



A'lus 



Fig. 102. — External anatomy of earthworm, ventral • view, segments in roman 

 numerals. (Prom Wolcott, Animal Biology, published by McGraw-Hill Book 

 Company, Inc.) 



mouth through its ventral side. In studying the earthworm it is 

 customary to number the segments with Roman numerals beginning 

 at the anterior end. This simplifies the study because external as 

 well as internal structures are definitely related to specific seg- 



