156 



BIOLOGY 



than in the younger ones, and increasing with age. Most 

 of these rings are alike in shape and size, 

 but a few of them differ slightly from the 

 others. The first one at the anterior end 

 is not a complete ring, but a minute pro- 

 jection which is known as the prostomium 

 (Gr. pro = before + stoma = mouth). It is 

 slightly movable and is the most sensitive 

 part of the animal. The second segment is 

 not a complete ring, but rather in the form 

 of a horseshoe, with the open part of the 

 horseshoe above, 

 and with the 

 prostomium lobe 

 fitting into the 

 opening as shown 

 in Figure 75. 



Underneath 

 the prostomium 

 and over the sec- 

 ond segment is 

 an opening, the 

 mouth, m. The 

 third segment is 

 a complete ring, 

 but ra ther small, 

 and from this 

 point backwards 

 the segments are 



all alike in shape, increasing slightly in size 

 until a maximum is reached, and from this 

 point remaining essentially the same in size 

 and shape to the posterior end of the body. 

 A short distance back from the head there 

 is a series of rings, from the twenty-eighth 



FIG. 74. AN 

 EARTHWORM, 

 FROM BELOW AND 

 FROM THE SIDE 



a, the anus; m , 

 mouth; od, opening of 

 oviduct; sr, opening of 

 seminal receptacles; v, 

 opening of vasdeferena; 

 a, aetse. 



FIG. 75. THE FIRST THREE 

 SEGMENTS OF THE EARTH- 

 WORM 



A, from the front; B, from the 

 side; p, the prostomial lobe; m, the 

 mouth. 



