This paper reviews the ultrastructural features and the behavioural aspects of presumptive chemoreceptors as reported in the literature, and adds some new observations. The involvement of both uniciliary and multiciliary sensory receptors has been cited. In some cases the expanded membrane area is due to microvillar projections and the ciliary segment is either reduced or absent. The chemoreceptive function can be inferred from comparison with similar, identified chemoreceptors in other taxa, from their specific location on the body surface, or from selective ablation experiments resulting in modified behavioural responses to known stimulants. In addition to feeding and ingestive behaviours, chemosensory cues are used in other ecologically adaptive functions such as habitat selection, intraspecific social communication which modulates reproduction, and several symbiotic relationships. Oriented behaviours can be altered by experience, giving rise to a more efficient colonization of different habitats or to a better exploitation of different food resources. Changes in water quality or the presence of pollutants will influence chemoreceptive abilities and can disrupt adaptive behavioural patterns.