Abstract
The general morphology and detailed histology of the penis of two common boreo-arctic cirripedes, Balanus balanoides (L.) and B. balanus (L.) have been investigated. The penis is a highly extensible, annulated organ beset with four rows of sensory setae. The paired vesiculae seminales unite within the pedicel of the penis to give the single ductus. Distally, the exoskeleton is invaginated into this ductus. Circular muscles are present in the vesiculae seminales but do not continue into the penis. The histology of the ductus epithelium indicates a secretory nature as does that of a specialized group of cells, termed the ‘cushion’, towards the distal end: this group of cells is surrounded by circular muscle bands. Longitudinal muscles extend virtually the whole length of the penis; they give off fibres which are inserted at the junctions of the annulations. The muscles of the pedicel are described. Paired nerves in the pedicel give rise to four in the penis. The sensory innervation of the setae is described. The possible functional relations of the structure to the activities of the penis at copulation and during the emission of semen is discussed.
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