The nervous system of Microstromum lineare (Turbellaria, Macrostomida). I. A fluorescence and electron microscopic study.

The nervous system (NS) of Microstomum lineare (Turbellaria, Macrostomida) was studied by electron and light microscopy, combined with fluorescence histochemistry (Falck-Hillarp method for biogenic monoamines). The NS is primitively organized, with a bilobed brain, two lateral nerve cords lacking commissures, and peripheral nerve cells scattered along the nerve cords. The stomatogastric NS, with a pharyngeal nerve ring, is joined to the central NS by a pair of connective ganglia. A green fluorescence in all parts of the NS indicates catecholaminergic neurons as the dominant neuron type. Ultrastructurally, two types of neurons were identified on the basis of their vesicle content: 1. Aminergic (catecholaminergic) neurons containing dense-core vesicles of varying electron-density and size, i.e., small dense-core vesicles (diameter 50--100 nm), vesicles with a highly electron-dense core (60--140 nm), and vesicles with an eccentric dense-core. 2. Presumed peptidergic neurosecretory neurons containing large granular vesicles (diameter about 200 nm) in the stomatogastric NS and peripheral parts of the central NS. In light microscopy, paraldehyde-thionin stained neurons were observed in the same areas.