INTRODUCTION THE EARLIEST DESCRIPTIONS of the gross organization of the leech nervous system go back to Poupart(48) and Cuvier (9). The first microscopic study of its cellular components was that of Valentin (66) and many further particulars of its organization were worked out by such discerning cytologists as Leydig (36), Hermann (21), Rhode (56), Apathy (l), and Retzius (51). It is the purpose of the present study to extend their observations to the level of the electron microscope and thus to provide a more detailed morphological basis for interpretation of the physiological properties of this nervous system. It was early established that the axial nervous system of this animal consists of a chain of 23 ganglia joined by connectives. The first and last ganglia are larger than the rest for they are formed by the fusion of several segmental ganglia early in development. The ganglionated chain is enclosed in a fibrous connective tissue capsule which is covered on the outside by a continuous sheet of flattened cells representing the visceral layer of the endothelium lining the ventral blood sinus that surrounds the entire central nervous system. There are three connectives between successive ganglia, two large lateral cords and a smaller median cord, called Faivre’s nerve (Fig. 1, A and B). Nerve roots emerge bilaterally from all the ganglia and quickly ramify into small nerve fascicles that carry fibers to and from the periphery. The bilateral nerve roots and the longitudinal connectives converge in a central mass of neuropil in the interior of each ganglion (Fig. 2C). This synaptic region is invested by a thin layer of connective tissue, called the inner capsule or the neurilemma, of earlier authors. Around the base of the nerve roots this layer is continuous with the thicker outer capsule that encloses the entire ganglion. Radially oriented fibrous septa connecting the inner and outer capsules of the ganglion in the midline dorsally and on either side ventrolaterally, are arranged so that they delimit six peripheral compartments which wil lbe referred to here as the “packets.” Each of these is occupied by a cluster of gan. glion ccl 1s and a single large glial cell, the glial cell of the packe t, which closely surrounds the cell bodies of the neurons and fills all the interstices between them (Figs. 2C and 3). The ganglion cells are ampulliform, l This investigation was supported by Grants GM 06729-05 and GM 10182 from the Institute of General Medical Sciences, Public Health Service.