FOOT OF NAIS. 



262 EVENINGS AT THE MICROSCOPE. 



the middle of which work a number of bristles (seta), 

 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 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 transparent as if drawn 

 out of spun glass. It is inter- 

 esting 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 thrust out and withdrawn in 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 

 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 

 body, a row of wartlike feet, from each of which project 

 two bundles of spines of exquisite structure. The bundles, 



