Monoamine‐containing neurons in planaria

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Abstract
The Falck and Hillarp fluorescence method was used in a study of the identification and distribution of monoamine‐containing neurons in three species of planarians, with principal attention given to Phagocata oregonensis. Nerve cells containing 5‐hydroxytryptamine (5HT) and others containing a primary catecholamine (CA) were found in the cerebral ganglia and ventral nerve cords; also in the peripheral plexus and pharynx. The fluorescing components of the peripheral plexus are mostly CA‐containing in dorsal regions and 5 HT‐containing in ventral regions. Some 5HT cells have processes that penetrate the ventral epidermis. These may be sensory or they may have a cilio‐regulatory role. A typical nerve net of multipolar CA cells was found in the pharynx; also a deeper layer of 5HT cells. The occurrence of other types of neurons (cholinergic and neurosecretory) in planarians is discussed. The early evolution of various common chemical types of nerve cells is indicated.