Ultrastructural observations on the phoronid nervous system.

Standard transmission electron microscopy (TEM) techniques have been applied to three species of phoronids to describe cytological features of their nervous systems. Four types of neurons have been distinguished by morphological and ultrastructural criteria, mainly involving their kinds of vesicles. Glial cells with characteristic polymorphic dense granules are abundant in the plexus, with possible roles in neuronal nourishment, mechanical support, and neurosecretion. Synaptic contacts and neuromuscular junctions across the connective tissue are described. The different structural features of the trunk giant axons in the three species mainly concern the enveloping sheath, and are described and discussed. Data on the innervation of internal organs, such as gut and nephridia, are also given. A detailed description of the structure, arrangement, and relationships of tentacular sensory cells, probably mechanoreceptors, is included, with a tentative hypothesis concerning the functions of these cells. © 1996 Wiley-Liss, Inc.