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Abstract

Morphology of the apical organ in Macracanthorhynchus hirudinaceus (Acanthocephala).

Abstract

The apical sensory organ of M. hirudinaceus was studied by electron microscopy. It is characterized by the interaction of apical sensory nerves and the sensory support cell duct. The 2 apical sensory nerves, 2 anterior proboscis nerves from the cerebral ganglion, and the duct from the sensory support cell extend anteriorly in the presoma in association with the proboscis retractor muscles. For most of the length of the proboscis, these 3 components are distinctly separate. The apical sensory nerves and sensory support cell duct enter the apical sensory organ near its posterior terminus and remain distinct but very pleomorphic for most of the length of this structure. The apical sensory nerves divide about 30 µm from the apex of the proboscis. Some of these branches become enclosed by the sensory support cell duct. The apical sensory nerves continue to divide and reunite until near the floor of the shallow pit at the anterior end of the proboscis. Those apical sensory nerves outside the sensory support cell duct terminate prior to the apical pit, whereas those surrounded by the duct form finger-like extensions filled with microtubules, microfilaments, and vesicles. The latter terminate at the floor and walls of the apical pit.