The Fine Structure of the Nervous System of the Cercaria of the Liver Fluke, Fasciola hepatica L.

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Abstract
The fine structure of neurons, nerve processes, synapses, neuromuscular junctions, and a previously undescribed sensory ending in the cercaria of Fasciola hepatica is described. The nerve processes (axons) are nonmyelinated and contain small mitochondria and vesicles and granules of several types. Some of the vesicles are similar to the synaptic vesicles common in other animals. Others show a marked similarity to the neuro-secretory vesicles described in both vertebrates and other invertebrates. Some problems of neurosecretion in the lower metazoa are discussed. Synapses and neuromuscular junctions are similar in structure to those described elsewhere. They show close contact of adjacent plasma membranes linked with increased density of the membranes and an asymmetric distribution of small vesicles. The sensory ending consists of a fine process which projects through the surface of the cercaria near the oral sucker, and is continuous proximally with a bulb which is part of a nerve fiber. From its structure and position, it is presumed to be a tangoreceptor.