188 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



distinguished by the rounded mouth which is just ventral to or under 

 a small protuberance, the 'prostomium, while the anus, or posterior end 

 of the digestive tract, is a tiny slit in the last segment. The posterior 

 end is also flattened, and between segments 32 to 37, not counting 

 the prostomium enclosing the mouth as the first, there is found a 

 swollen region, called the clitellum, important in reproduction. 



The upper or dorsal side may be distinguished by its darker color, 

 while the ventral side is slightly flattened and contains four double 

 rows of tiny projections called setae, which give the worm a grip on 

 the ground when in locomotion. The dorsal side is devoid of any 



The common earthworm, Lunibricus ierresiris. 



Wright Pierce 



Note the swollen area, or clitellum. 



openings except some very minute dorsal pores that communicate 

 with the body cavity, or coelom, but the ventral side has several 

 paired openings, difficult to find, which lead to the reproductive and 

 excretory organs. The surface of the body is covered with a delicate 

 iridescent cuticle, secreted by the living epithelial cells of the skin, but 

 which is itself dead. Its iridescence is caused by the presence of 

 numerous grooves (striae), and its surface is pierced with small holes, 

 which are openings for the mucous gland cells of the skin. The coelom 

 or body cavity is cut up into small compartments by partition walls, 

 or septa, that are absent or incomplete in the extreme anterior region, 

 between the 18th and 19th segments, and in the region posterior to 

 the reproductive organs. The coelom in the living worm is filled 



