246 THE MECHANICAL FUNCTIONS. 



Annelida^ some provision of this kind, often con- 

 sisting of tufts of hair regularly disposed in rows 

 on each side of the under surface. In the Nereis 

 (Fig. 129), a genus of sea- worms, there are often 

 above a hundred pair of little tufts of strong bristles : 

 and between these we find tentacula to prevent 

 the animal from running against any thing by 

 which it might be injured. They also raise the 

 body from the ground, for effecting which, as they 

 are used under water, very little support is neces- 

 sary.* 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 description 

 advances along the ground is very simple. It first 

 protrudes the head by the elongation of the fore- 

 most segments of the body, while the other segments 

 cling to the earth by means of the rings, and also 

 of the bristles and other appendages to the integu- 

 ments. 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 



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



