Morphology and behaviour of the benthic larva of Arenicola cristata (Polychaeta)

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Abstract
The structure of the three-setiger larva of Arenicola cristata is described with emphasis on the ultrastructure of the apical region, particularly the nervous and muscular systems, and ciliated and glandular cells. At least one population of well-differentiated nerve cell bodies can be identified in the cerebral ganglia, and these are filled with dense-cored vesicles. Processes apparently derived from these cells, containing both dense-cored and clear vesicles, are found throughout the cerebral commissure and circumesophageal connectives. Nerve terminals are restricted to a single small region along each of the two ventrolateral longitudinal muscles. Most of the terminals contain predominantly one or the other of the two vesicle types, but some terminals appear to contain both. Normal behaviour involves control of both muscular and ciliary activities. Experiments with drugs indicate a muscarinic cholinergic innervation of muscle, but we have found neither pharmacological nor structural evidence for a mechanism controlling ciliary activity. Preliminary observations suggest the presence of biogenic amines exhibiting specific fluorescence and a relationship between serotonin and the secretion of mucus.

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