BORERS. 303 
matter are illuminated while the back-ground is 
dark, and these by their motion clearly reveal the 
currents of the fluid in which they are suspended. 
A few moments’ practice will enable even an un- 
accustomed eye to perceive the atoms converging 
from all points around, with an even but increasing 
velocity, towards the principal tube, down which 
they disappear like the streams of passengers and 
traffic in the neighbourhood of a great city, con- 
verging towards it as to a common centre of 
attraction by a hundred different routes. The 
current of the expelling tube is even still more 
marked in its character; a forcible jet of water is 
periodically ejected from this orifice, which draws 
the surrounding particles into its vortex, and 
shoots them forward to a distance of many inches. 
It is by the expulsive force of this anal current, 
chiefly, that the passage is kept free from the 
deposit of mud and other substances, which would 
otherwise soon choke it up. 
A fresh supply of water for respiration, and its 
dismissal when no longer fit for use, are efficiently 
provided for by this contrivance. But since many 
particles of matter float in the water, which from 
their form or other qualities might be hurtful to 
the delicate tissues of the viscera to be traversed, 
how is the entrance of these to be guarded against 
in an indiscriminating current? A beautiful con- 
trivance is provided for this necessity. The mar- 
gin of the entering siphon, and sometimes, though 
more rarely, of the ejecting one, is set round with 
a number of short tentacular processes, varying 
indeed in their length, but the longest scarcely 
more than equalling half the diameter of the mouth 
of the tube. In Sazicava rugosa, which bores 
