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IOWA ACADEMY OF SCIENCES. 
either of the other ganglia, when viewed from the side. They 
lie beneath and a little posterior to the intestinal coil at the 
line where the muscles of the foot come in contact with the con- 
nective tissues of the body proper, Fig. 4. The pedal ganglia 
are likewise fused together by their adjacent sides. 
The cerebral ganglia are connected. Fig. 1, with the parieto- 
splanchnic ganglia by the cerebro- visceral commissures and 
with the pedal ganglia by the cerebro- pedal commissures. 
Beside these commissural connections each cerebral ganglion 
gives rise to a small nerve which supplies the anterior adduc- 
tor muscle and a larger nerve which passes down behind the 
anterior adductor muscle into the mantle and supplies the 
pallial muscles of its anterior portion. 
Each parieto- splanchnic ganglion besides its commissural 
connection, gives rise to a small nerve which supplies the pos- 
terior adductor muscle, a larger branchial nerve which runs 
forward a short distance, passes over into the junction of the 
outer lamella of the inner gill with the inner lamella of the 
outer gill, where it turns abruptly backward and apparently 
• ends at the posterior ends of the gills not greatly reduced in 
size, and a large nerve that runs around the ventral surface of 
the posterior adductor muscle and branches. The smaller 
branch is probably distributed to the muscles of the siphons, 
but I have been unable to follow it far. The larger branch 
runs along the mantle near the inner ends of the pallial 
muscles, giving off a branch near the upper border of the 
branchial siphon and numerous small branches to the pallial 
muscles. 
Each pedal ganglion, besides its commissural connection, 
gives rise to at least five more or less distinct nerves which 
branch among the muscles of the foot. 
OTOCYSTS. 
A pair of otocysts, Fig. 1, o lie directly in front of the 
pedal ganglia, almost, if not quite in contact with the cerebro- 
pedal commissures. They are nearly spherical in shape, and 
consist of a wall of cells with a nearly spherical otolith inside 
(Nos. 4 and 5). Thus far I have been unable to find cilia in the 
otocysts, but this may be the fault of preservation. The 
otocysts of most Lamellibranches are described as being ener- 
vated by fibres from the cerebro-pedal commissures. With 
Sphasrium a small branch is given off from the nerve which 
passes immediately below each otocyst that passes up, and may 
