Ultrastructure of the Anterior End in Three Ontogenetic Stages of Nectonema munidae (Nematomorpha)

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Abstract
The structure of the anterior end of three ontogenetically successive stages of Nectonema munidae (Nematomorpha) is investigated by light and transmission electron microscopy. During development, an adult cuticle is formed under a larval cuticle, which is subsequently moulted. Only one moult can be documented for Nectonema. The brain has a main subpharyngeal portion and a weak suprapharyngeal commissure. It is circumpharyngeal only in early developmental stages, the dorsal commissure is reduced in the adult stage. Four giant cells and a cerebral cavity are adult features. Although the morphology of the giant cells is elucidated, their function remains unclear, but a sensory function is probable. A septum marks the posterior border of the anterior end and divides a cerebral cavity from a body cavity. A precursor of the septum is present in the first stage observed, but it lies next to the epidermis and does not separate a cerebral cavity. Cuticular structures in the pre‐pharyngeal region of the early stages are interpreted as remnants of the larval boring organ. They are moulted together with the larval cuticle. The morphology of the pharynx and the anterior part of the intestine is shown.