Ultrastructure of the Head of Okranema eileenae Greenslade and Nicholas, 1991 (Thoracostomopsidae: Nematoda).

Okranema eileenae is a marine nematode from Australian sandy beaches. The structure of its cephalic region is described by light, scanning, and transmission electron microscopy. Three large lips, separated by three deep clefts and surmounted by flexible liplets, surround the mouth. Transverse and longitudinal sections of the head have been used to investigate the ultrastructure of the lips, buccal cavity, mandibles, and cephalic sensilla. The pharyngeal muscles are attached to the external head cuticle over a broad band, forming the cephalic capsule, which terminates in the three connected cuticular cephalic arches-one in each lip. Three mandibles form the central core of each lip, cuticular structures from which two small teeth-denticles and one large central tooth, the onchium-project into the buccal cavity. The onchia are anterior extensions of the pharynx that contain the ducts of pharyngeal glands that discharge into the buccal cavity. Epidermal tissue extends anteriorly as far as the cephalic arch. Cephalic structures, apart from sensory setae, are formed from an expanded cephalic cuticle.The inner labial, outer labial, and cephalic setae each contain two dendritic processes. Aspects of amphidial structure are described for the first time in Thoracostomopsidae. The external apertures are illustrated by scanning electron microscopy and the internal structure in sections by transmission electron microscopy. Two bundles of about 100 dendritic processes are enclosed by the amphidial sheath cell, as well as a group of four other dendritic processes. Two amphidial duct cells are present on each side of the head, but without containing dendritic processes. However, the ultrastructural description of the amphids is incomplete.