262 EVENINGS AT THE MICROSCOPE. 



pairs of "feet" on each ring, consisting of wart-like pro- 

 minences, 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 

 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. It is interesting to ob- 

 serve with what lisjhtnin^-like 



FOOT OF NAIS. ° 



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 and withdrawn in constant succession, as the body is 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 

 marine one. Here is a Polynoe, a curious genus, very 

 common under stones at low water on our rocky shores. 

 It is remarkable on several accounts. All down the back 

 we discover a set of oval or kidney-shaped plates, which 

 are called the back shields {dorsal elytra) ; these are flat, 

 and are planted upon the back by little foot-stalks set on 

 near the margin of the under surface ; they are arranged 

 in two rows, overlapping each other at the edge. These 

 kidney-shaped shields, which can be detached with slight 

 violence, are studded over with little transparent oval 

 bodies, set on short foot-stalks which are perhaps delicate 

 organs of touch. The intermediate antennae, the tentacles, 

 and the cirri, or filaments of the feet, are similarly fringed 

 with these little appendages, which resemble the glands 

 of certain plants, and have a most singular appearance. 

 If we remove the shields, we discover, on each side of the 



