274 THE MECHANICAL FUNCTIONS. 



wholly elongated, according as the actions of 

 these oblique muscles are partially or generally 

 exerted. 



The skin on the surface of the earth-worm is 

 furnished, at the parts where it covers the rings, 

 with very minute bristles, called Setce, by means 

 of which the animal is enabled to fix those parts 

 on the ground, while the other portions of its 

 body are in motion. Both in the anterior and 

 posterior segments, these hairs are directed to- 

 wards the centre of the animal ; while those on 

 the middle segments are perpendicular.* We 

 almost constantly find, in animals belonging 

 to the order of Annelida, some provision of this 

 kind. Often it consists of tufts of hair regu- 

 larly disposed in rows on each side of the under 

 surface. In the Nereis (Fig. 1*29), a genus of 

 sea-worms, tliere are often above a hundred 

 pair of little tufts of strong bristles: and be- 

 tween 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 which purpose, as they 



* As an instance of the extraordinary mnltiplicity of species 

 existing in every department of living nature, I may here notice, 

 that of the common earth-worm, apparently so uniform in its 

 shape, Savigny has lately, by a closer examination, been able to 

 distinguish no less than twenty-two different species, among 

 those found in the neis:hbourhood of Paris alone. 



