64 -SIPHONAL CURRENTS. 



and departing currents keep tliis passage clear, a pro- 

 cess which in mud or sand miglit seem at first not 

 very easy of accomplishment. It is facilitated, how- 

 ever, hy the faculty which the boring bivalves have 

 of lengthening the siphonal tubes at will ; and the 

 degree to which this may be accomplished depends 

 on the depth of the cavity which the species is ac- 

 customed to make. 



If we take one of the stone-boring Mollusca, a 

 Pholas or a Saxicava for example, from its excava- 

 tion, without injuring the animal, and place it in a 

 glass vessel of sea- water, it will not be difficult to de- 

 tect the currents in question, even with the naked 

 eye ; though a lens of moderate power will render 

 them more distinctly appreciable. The vessel should 

 be so placed as that the light may be nearly, but not 

 exactly, opposite to the eye. By this arrangement 

 the minute atoms of floating 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 unaccustomed 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, converging 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 continuously 

 ejected from this orifice, which draws the surrounding 

 particles into its vortex, and shoots them forward to a 



