jyQ EARTHWORMS. PHYLUM ANNELIDA 



longitudinal muscles contract the chsetae are generally simultane- 

 ously extended. As those of the first dozen or so segments point 

 backwards they will resist backward movement more than 

 forward ; those of the middle part of the body stand out radially, 

 or slightly irregularly, so that they will resist forward and 

 backward movement equally. Those of the posterior segments 

 point strongly forwards, and if erected would tend to make the 

 worm move backwards ; as has been said, the contraction waves 

 do not normally reach this part of the body, and so the chsetae are 

 not extended. Finally, and probably most important, the anterior 

 end of the body acts as a sucker. The prostomium and peris- 

 tomium are pressed against the surface, and the prestomium 

 is then withdrawn ; when this happens the anterior end is 

 anchored, and any shortening of the body must pull the animal's 

 centre of gravity forward. By this means a worm can crawi up a 

 vertical plate of glass or metal. 



Burrowing takes place by two methods. If, when the anterior 

 end of the body is elongated, it comes into any small crevice, 

 instead of contracting, as it normally would do on contact, it 

 continues to press in. Then when the wave of longitudinal con- 

 traction starts, the anterior segments expand radially and press 

 against the sides of the crevice so that they cannot be withdrawn. 

 By this means, which is akin to the technique of chimney- 

 climbing known as backing-up, a worm can move through a 

 glass tube, and it is probable that much of its burrowing is done 

 in this way. In loose soil a more or less cylindrical burrow will 

 be formed by the pressure of the segments against the side of 

 the crack into which the worm puts its snout. Sometimes on 

 contact with soil the worm extends its pharynx and sucks in 

 a mouthful ; repeated sucking creates a hollow in which the 

 backing-up process can be used. A little soil is sometimes swallowed 

 without protrusion of the pharynx. It is probable that burrowing 

 in firm soil is impossible unless the worm eats its way through. 



When a worm is strongly stimulated it may move backwards 

 by a reversal of the normal waves. It is probable that this 

 reaction seldom occurs on the surface in nature, but it is important 

 in preventing the worm from being withdrawn from its burrow. 

 The longitudinal muscles of the posterior segments are strongly 

 contracted, with extrusion of their chaetae. As has been said, these 

 project forwards, and if this part of the worm is below ground 

 thev will be driven into the soil. If the anterior end is seized 



