ANNELIDA. 275 



are used under water, very little support is ne- 

 cessary.* Sometimes the whole body is covered 

 with hair ; at other times these appendages are 

 in the form of hooks, which, of course, give 

 greater power of clinging to the objects on which 

 they fasten. In some, again, they assume more 

 the nature of feet, of which they exercise during 

 progression all the functions ; being furnished 

 with several sets of muscles for adjusting and 

 strengthening their actions. 



The mode by which an animal of this de- 

 scription advances along the ground is very 

 simple. It first protrudes the head by the elon- 

 gation of the foremost segments of the body, 

 while the others cling to the earth by means 

 of the rings, and also of the bristles and other 

 appendages to the integuments. The head 

 is then applied to the ground, and made the 

 fixed point, and the segments next to it, which 

 had been elongated, are now contracted by 

 the action of their longitudinal muscles ; in 

 doing which, equal portions of the succeeding 

 segments are necessarily elongated : these are 

 next contracted ; and so on, in succession, till 

 the whole is brought forwards to the head : after 

 which the same series of actions is repeated, 

 beginning with the advance of the head. 



* Home; Lectures, &c. Vol. i. p. 115. 



