16 LAND AND FRESH-WATER MOLLUSKS. 
holes, which are beset with vibratile cilia ; by the 
action of these extremely delicate filaments, a - 
current of water is produced, setting im by the 
branchial orifice (fig. 1, b), which is surrounded 
by a fringe of sensitive tentacular organs, and 
sweeping over the gills, it is propelled towards 
the mouth (0), and passes out by the anal orifice 
(v) : by this agency, the layer of water in con- 
tact with the gills is thus renewed, and food is 
also brought to the mouth. 
These currents may be rendered distinct by 
introducing some powdered indigo into a basin © 
of water containing a fresh-water mussel. 
The branchial currents are most conveniently 
exhibited by the Zebra mussel, possessing, as it 
does, two prominent siphons (fig. 2). The in- 
halent siphon is densely fringed, and if the pro- — 
cesses be touched, the orifice closes, and the 
siphon is at once retracted. When unmolested, 
a current flows steadily into the one orifice, whilst 
another current rises up from the other, as in- 
dicated by the arrows. 
The wonderful activity of the cilia is a most 
striking spectacle ; a magnifying power of about 
120 diameters is amply sufficient to obtain a — 
general view of the movements. A small portion 
of the gill must be carefully spread out on a 
glass slide, with a little water, and covered by a 
glass disk ; or the animalcule-cage may be con- 
