306 PHYLUM MOLLUSCA. 
and leaves a furrow in its track. The muscle fibres, as in 
the snail, are mainly of the slowly contracting non-striped 
sort, but those of the adductor and of the heart show 
oblique cross-striping. In that part of the adductor muscle 
of Pecten (and some other bivalves) that effects the rapid 
closing of the valves, and hence the swimming, the muscle- 
fibres are transversely cross-striped, and the same is true of 
those found in the margin of the mobile mantle. There is 
here therefore a good instance of the connection between 
striation and rapidity of contraction and relaxation. 
Nervous system.—There are three pairs of nerve- 
centres :— : 
(a) Cerebro-pleural ganglia, lying above the mouth on 
each side on the tendon of the anterior retractor 
of the foot, connected to one another by a 
commissure, connected to the two other pairs 
of ganglia (4) and (c), by long paired connect- 
ives, and giving off some nerves to mantle, 
palps, etc. 
(2) Pedal ganglia, lying close together about the 
middle of the foot, united by connectives to (a), 
giving off nerves to the foot, and having beside 
them two small ear-sacs, each with a calcareous 
otolith, and with a nerve said to be derived 
from the cerebral ganglion. 
(c) Visceral ganglia (also called parieto-splanchnic or 
osphradial), lying below the posterior adductor, 
connected to (a) by two long connectives, and 
giving off nerves to mantle, muscles, etc., and 
to a patch of “smelling cells” (osphradium) at 
the bases of the gills. 
Sense organs.—Unlike not a few bivalves, which have 
hundreds of “eyes” on the mantle margin, Azodonta has 
no trace of any. The ear-sac, originally derived from a skin- 
pit, is sunk deeply within the foot, and is of doubtful use. 
The ‘‘smelling patch” or “ osphradium” at the base of the 
gills has perhaps water-testing qualities. There are also 
“tactile” cells about the mantle, labial palps, ete. 
Alimentary system.—The mouth lies between the 
anterior adductor and the foot, and beside it lie the ciliated, 
vascular, and sensitive labial palps, two on each side, which 
