266 EVENINGS AT THE MICROSCOPE. 



you have often seen it in the little hollows of our rocky 

 ledges, and especially on beds of young mussels ; and pro- 

 bably you have admired the elegant ease with which its 

 lithe and tortuous body writhes and winds, like a bit of 

 green silken cord, in and out among the compactly 

 crowded shells. You have wondered, too, at the difficulty 

 which attends the attempt to take it up, not on account of 

 the rapidity of its motions, but because of the extreme 

 slenderness and slipperiness of the subject, and of the 

 power which it possesses of insinuating itself into the 

 smallest crevice. 



The foot in this genus has but a single branch, and a 

 single pencil of bristles, which is placed between the flat 

 swimming leaf that ornaments each segment and the lower 

 cirrus. The bristles are of the compound jointed form, 

 but the joint is fixed in a peculiar manner. The basal 

 portion is drawn out into a very slender long straight 

 shaft, terminating in a knob somewhat resembling the end 

 of a limb-bone. This is slit in one direction to receive the 

 terminal piece, which is shaped somewhat like a lance- 

 head, and is inserted into the slit exactly as a knife-blade 

 is fixed into the haft. The head is in fact a knife-blade, 

 with a thickened back and a very thin edge, which is 

 notched into teeth of the most exquisite delicacy. The 

 blade is slightly curved, and drawn out into a long acute 

 point ; and the whole bristle is formed out of an elastic 

 horny substance (probably chitine), that rivals in transpa- 

 rency and brilliancy the purest flint-glass. 



I might adduce a vast variety of examples of these 

 organs in the Marine Worms, all of which would charm 

 you by their elegance and by their extreme diversity ; but 

 I have other things to show you in this interesting class of 

 animals, which fortunately are so common on all our 

 shores that you will have no difficulty in procuring plenty 

 of specimens for your private observation and study. And 

 if you need intelligent guidance you cannot have a better 



