178 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 iuto 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 oft 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 t, 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 

 Sphserium a small branch is given off from the nerve which 

 passes immediately below each otocyst that passes up, and may 



