133 LIFE, IN ITS INTERMEDIATE FORMS. 



ifhat singular stopper, of which the function is so mani- 

 fest, is one of a pair of tentacles ; organs which in general 

 are exactly alike. Here, however, one is destined to close 

 the orifice, and as one only could perform that office, the 

 other is quite plain, a simple thread, while this is enlarged 

 iuto a conical plug. What wise contrivance is manifested 

 here ! 



We wondered at the extraordinary rapidity with which 

 the timid animal disappeared on alarm, and are curious 

 to know the mechanism by which it is effected. Each of 

 the rings of which the body is composed carries on each 

 side a little wart-like foot, within which is a bundle of 

 horny bristles, like the filaments of a hair-pencil, capable 

 of being protruded and withdrawn. The microscopic 

 structure of these is most elaborate, but we cannot detail 

 it here ; it may be sufficient to say that it is by the pro- 

 trusion of these pencils in turn, which press backwards 

 against the sides of the tube, that the animal pushes its 

 foreparts out. 



But the retreat requires a more powerful machinery for 

 its extraordinary fieetness ; and this deserves a more close 

 investigation. On carefully examining a Serpula recently 

 dead, we observe, by means of 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 ex- 

 cessively delicate membrane, and on placing it under a 

 high power (say 300 diameters) we are astonished at the 

 elaborate provision here made for prehension. This yellow 

 line, which cannot be appreciated by the unassisted eye, 

 is a small muscular ribbon, on which stand up edgewise a 

 multitude of what we mav call combs, or rather sub- 



