138 LIFE, IN ITS INTERMEDIATE FORMS. 
That 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 
into a conical plug. What wise contrivance is manifested 
here 1 
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of being protruded and withdrawn.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 fleetness ; and this deserves a more close 
investigation. On carefully examining a Sorpula recently 
dead, wc observe, by means of a Ions, 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 wo may call combs, or rather sub- 
