298 EVENINGS AT THE MICROSCOPE 



the greatest apparent ease through substances which it 

 would 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 sim- 

 plest form: it is to the aquatic Worms that you must look 

 if you wish to see the amazing diversity, complexity, and 

 delicacy of these organs. In these there are one or two 

 pairs of 4t feet" on each ring, consisting of wart-like prom- 

 inences, which are perforate and protrusile, and through 

 the middle of which work a number of bristles (setoe), ar- 

 ranged in a radiating pencil, something like the hairs of a 

 paint-brush. In this transparent and colorless little Nais 

 from fresh water you may see their form and arrangement; 

 in complexity they present an advance upon the Earth- 



worm, for here there are some seven 

 or eight bristles in each pencil, which 

 radiate in the same plane, and are 

 graduated in length; they are very 

 slender, bent at the tip, and as trans* 

 parent as if drawn out of spun glass. 

 FOOT OP NAIS. it is interesting to observe with what 



lightning-like rapidity they are thrust out and withdrawn 

 in constant succession, as the body is ever lengthening 

 and shortening. 



Let us exchange this little fresh-water Worm for a ma- 

 rine one. Here is a Polynoe, a curious genus, very com- 

 mon under stones at low water on our rocky shores. It 

 is remarkable on several accounts. All down the back we 



1 "Gen. Outline," p. 202. 



