Ultrastructure of the acanthella of Paratenuisentis ambiguus (Acanthocephala).

The fine structure of the early acanthella of Paratenuisentis ambiguus (Eoacanthocephala) was investigated. This developmental stage is characterised by losing its ability to move and by differentiation of adult structures. The frontal syncytium, present in the first developmental stage (the acanthor), is lost, while the epidermis and central syncytium persist. The epidermis of the acanthella contains a number of giant nuclei that are arranged into several small groups. The central syncytium is subdivided into different masses, containing nuclei that will give rise to the organs of the adult. The 'uncinogenous bands' extend into the anterior body of the acanthella. Formation of the hooks takes place within these strands. In all investigated stages no extracellular materials were observed. Posterior of the uncinogenous bands lies the brain anlage and the primordia of the reproductive system. Neither a sense organ nor a nervous system were found.