Nervous System

The nervous system of the oyster is relatively simple. The visceral and cerebral ganglia are joined by the cerebro-visceral connectives; the U-shaped cerebral commissure goes a.round the esophagus; the circumpallial nerve travels along the mantle's edge; and a number of nerves originate from the ganglia and extend to different parts of the body. The pedal ganglia, well developed in many other bivalves, and the cerebropedal connective are absent. These retrogressive features are associated with the sedentary mode of life of the oyster and the loss of the organ of locomotion (foot). In the evolution of bivalves this simplification of the anatomy represents an adaptive cha.nge and cannot be regarded as a primitive trait (Jhering, 1877). The only organs of sense in the oyster are the tentacles, along the edge of the mantle, and the pallial organ inside the cloaca.. The tentacles are highly sensitive to changes in illumination; they contract if a sha.dow passes in front of a feeding oyster, or II. beam of light is focused on them. They also detect the presence of minute quantities of various drugs, chemicals, excessive amounts of suspended particles, and changes in temperature and composition of sea water. The function of the pallial organ is not well understood; t~e organ is probably concerned with the detecbon of mechanical. disturbances in the surrounding water. The eyes found in freely mo\;ng bivalves, such as scallops, are absent in adult oysters but are present in fully grown larvae. . Th~re is no major control center (brain), and m th.lS Bense the nervous system is decentralized. The mtegration of its various partEl is accomplished by the interconnections of the ga.nglia through