WOEMS. 301 



the spines upon the hinder rings then take a firm hold 

 upon the sides of the hole thus formed; and, preventing 

 any retrograde movement, the head is again forced for- 

 ward through the yielding mould ; so that, by a repeti- 

 tion of the process, the animal is able to advance with 

 the greatest apparent ease through substances which it 

 woiald at first seem utterly impossible for so helpless a 

 being to penetrate." * 



Implements analogous to these are found in most 

 of the animals of the class Annelida, to which the 

 Earth-worm belongs. But in this creature you see 

 them in their simplest form : it is to the aquatic 

 Worms that you must look if you wish to see the amaz- 

 ing diversity, complexity, and delicacy of these organs. 

 In these there are one or two pairs of " feet" on each 

 ring, consisting of wart-like prominences, which are 

 perforate and protrusile, and through the middle of 

 which work a number of bristles {setce), arranged in a 

 radiating pencil, something like the hairs of a paint- 

 brush. In this transparent and colourless little Nais 

 from fresh water,- you may see their form and arrange- 

 ment ; in complexity they present an advance upon 

 the Earth-worm, for here there 

 are some seven or eight bris- 

 tles in each pencil, which radi- 

 ate in the same plane, and are 

 graduated in length ; they are 

 very slender, bent at the tip, 

 and as transparent as if drawn 

 out of spun glass. It is inter- 

 esting to observe with what lightning-like rapidity thoy 



* Gen. Outline, p. 202. 



FOOT OP NXIS. 



