Publisher Summary This chapter discusses the morphological, neurophysiological, endocrinological, biochemical, and pharmacological characteristics of the caudo-dorsal cells (CDC), with particular reference to structure–function relationships. Three main factors have contributed to the present knowledge of the nature of the peptidergic CDC of Lymnaea stagnalis: the suitability of the cells for experimental studies, the multidisciplinary approach with which they have been investigated, and the detailed knowledge of various related aspects of the snail's biology, such as endocrinology, reproduction, the sensory system, the muscular system, the blood and connective tissue, and hydromineral regulation. Some of the results may increase the insight into structure and functioning of nervous systems in general. The quantitative electron microscopic analyses of the cyclic dynamics of CDC activity show the way in which a neuroendocrine system can meet the physiological demand for a sudden high titer of neurohormone to induce ovulation by synthesizing, transporting, and degrading secretory material in a timed, cyclic, and quantitative fashion. The clear relation between the electrical discharge and massive release of ovulation neurohormone of Lymnaea stagnalis (CDCH) demonstrates the significance of the simultaneous neural and endocrine character of the neuroendocrine cell.