Molluscs show an enormous diversity in level of organisation and habit of life. Some have a simple nervous system organized like that of flat- worms, while others, viz., the cephalopods, have a highly developed nervous system comparable with that of vertebrates. The functional aspects of the cephalopod central nervous system (CNS) have been extensively investigated. Reviews of these studies can be found in WELLS (1962, 1966) and YOUNG (1964). Other groups have been studied less thoroughly. Bivalves have at- tracted attention especially with regard to problems of control and mechanism of prolonged contractions of adductor muscles, and of rhythms in shell movements (see HOYLE, 1966). The cerebral control of the adductor muscles was studied by PAVLOV (1885) and BARNES (1955) in Anodonta cygnaea.