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the most delicate and close -set teeth, the lines of which 

 pass back upon the blade, as in our reaping-hooks. 



These pencils of spear-like bristles are the organs by 

 which the protrusion of the animal is performed. Their 

 action is manifestly that of pushing against the walls of 

 the interior, which on close examination are seen to be 

 lined with a delicate membrane, exuded from the animal's 

 skin. The opposite feet of one segment protrude the pen- 

 cils of bristles, one on each side, the acute points and teeth 

 of which penetrate and catch in the lining membrane; the 

 segments behind this are now drawn up close, and extend 

 their bristles ; these catch in like manner ; then an elongat- 

 ing movement takes place ; the pencils of the anterior seg- 

 ments being now retracted, they yield to the movement 

 and are pushed forward, while the others are held firm by 

 the resistance of their holding bristles; thus gradually the 

 fore parts of the animal are exposed. 



But this gradual process would ill suit the necessity of 

 a creature so sensitive to alarm, when it wishes to retreat. 

 We have already seen how, with the fleetness of a thought, 

 its beautiful crown of scarlet plumes disappears within its 

 stony fastness: let us now look at the apparatus which 

 effects this movement. 



If you look again at this Serpula recently extracted, 

 you will find, with a lens, a pale yellow line running along 

 the upper surface of each foot, transversely to the length 

 of the body. This is the border of an excessively delicate 

 membrane; and on placing it under a higher power (say 

 600 diameters), you will be astonished at the elaborate pro- 

 vision here made for prehension. This yellow line, which 

 cannot be appreciated by the unassisted eye, is a muscu- 

 lar ribbon over which stand up edgewise a multitude of 



