The 34 ganglia of the leech (Hintdo medicinalis) central nervous system together with their connectives and roots have been studied by means of the 5uorescence histochemical technique for monoamines.More than half the ganglia of the central nervous system possess the same number of monoamine-containing neurons in a fixed location. These identical ganglia consist of seven yellow 5uorescent monoamine-containing neuron perikarya located within the ganglia and a pair of blue-green fluorescent monoamine-containing perikarya located outside the ganglia in the anterior roots. A neuropile consisting of a dense network of yellow and blue-green fluorescent varicose processes, several smooth blue-green fluorescent processes and occasional smooth yellow fluorescent processes is present in the ganglia. Smooth and varicose yellow and blue-green fluorescent processes are found in the connectives and roots of the ganglia. The intraneuronal distribution of fluorescence within the nine monoamhe-containing neurons of identical ganglia is described.The monoamine-containing neurons of the remaining ganglia of the central nervous system are described. These ganglia are shown to vary only slightly from the pattern of identical ganglia. 1This research was sup orted by a U. S. Public Health post-doctoral fellowskp (2F2NB 34,824-01.02). ZSome of the results reported here apyear in abstract form: Rude, S. 1967 Am. Zoolosst, 7: 738.

The two largest cells in a typical ganglion of the leech (Hirudo medicinalis) nervous system are the colossal cells of Retzius . These cells show a positive chromafftn reaction,

This study represents an attempt to reconcile recent quantitative findings of large amounts of monoamines in the earthworm with early histological findings of only small numbers of chromaffin cells in that animal. Using the highly sensitive and specific fluorescence method for the histochemical localization of monoamines, it has been demonstrated that the earthworm nervous system contains large numbers of monoaminergic neurons. At least one-tenth of the motor and interneuronal population i n a typical ganglion of the ventral nerve cord contain either serotonin or primary catecholamines. In addition a large number of neurons containing a primary catecholamine form an extensive sensory system with their cell bodies in the epidermis and their axons terminating in the ventral nerve cord.

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