EARTHWORMS 165 



where. The precise use of these special bristles is not fully 

 known, but it is probable that they are of use in working the 

 cocoon off the body, and during the act of pairing. 



The alternate elongations and contractions of the body are 

 brought about by two sets of muscles which lie immediately 

 below the skin and completely invest the animal all round. The 

 outer set is arranged as a series of hoops round the body, the 

 inner runs lengthways from head to tail. When the former are 

 used they squeeze the body so as to make it thinner and longer ; 

 at the same time they stretch out the other, longitudinal set of 

 muscles ; when these latter come into play they reverse the 

 effect of the circular set, drawing the head and tail closer together, 

 so that the worm becomes shorter but fatter. The action of the 

 circular set may roughly be imitated by grasping a roll of putty 

 in the hand and closing the fingers tightly ; the pressure so applied 

 will cause the roll to become longer and thinner. If the putty 

 were enclosed in a piece of elastic indiarubber tubing which was 

 blocked up at each end, the pressure of the hand would simul- 

 taneously stretch out the tubing ; on removal of the pressure the 

 elastic material would resume its previous shorter condition, and 

 so afford a fair representation of the behaviour of the longitudinal 

 muscles. It is interesting to note that the longitudinal set is 

 many times thicker and stronger than the circular ; the value of 

 the difference is evident when it is remembered that a worm's 

 life often depends upon the speed with which the body can be 

 shortened and withdrawn backwards into the burrow upon the 

 approach of danger. Ability to lengthen itself rapidly is never 

 of vital importance. 



Defence. To a soft-bodied creature, such as a worm, discreet 

 retreat is undoubtedly the better part of valour ; but the animal 

 is not entirely destitute of means of self-defence. The slime 

 with which the whole surface of the body is covered renders the 

 worm slippery and difficult to hold, so that escape is often accom- 

 plished from the beak of the thrush and other birds. But a large 

 number of the foes of the worm are microscopic parasites, both 

 plant and animal, which abound in the rich upper soil that is 

 chiefly frequented by worms. Many of these minute enemies 

 are caught and smothered in the slime, while some are actually 



