The development of the body wall ofPolymorphus minutus(Acanthocephala) in its intermediate hostGammarus pulex

Abstract

The ultrastructure and development of the body wall ofPolymorphus minutusis described during the acanthella stages in the intermediate hostGammarus pulex.The cortex of the early acanthella is simple in structure, consisting of a syncytium containing giant nuclei, mitochondria, Golgi bodies, endoplasmic reticulum and a few lipid bodies. The cuticle is penetrated by pores which open into vesicles in a vesicular region.The cortex gives rise to the body wall, neck and lemnisci of the adult, and may be considered as divided initially into two and later three functionally different regions: an outer absorptive region, an inner storage region and a layer between which develops to provide firm, resistant skeletal protection.Absorption is facilitated by the increase in surface area due to the pores which penetrate the surface. This region is functional throughout development and the pores increase in number, possibly by division, as the animal grows. Beneath the pores a pore canal layer develops towards the end of the late acanthella stage.During development supporting fibres are deposited throughout the cortex. Beneath the vesicular layer a concentration of mitochondria, Golgi bodies and endoplasmic reticulum deposit the fibres of the felt layer. In the anterior meta-soma no felt layer is found, but the body spines originate from this region.The inner region of the cortex, or radial layer, contains the giant nuclei, which are arranged in the early acanthella in 5 rings of 4, 6, 5 or 6, 6 and 5 nuclei respectively. During development they produce dendritic process and finally fragment. They are considered here to be concerned with the formation of the felt layer and lemnisci rather than the ‘lacunar channels’. The inner cortical region becomes ensely packed with lipid bodies, and it is suggested that these may be responsible, owing to fixation artifact, for the so-called ‘lacunar channels’ previously described in these regions, the extensive cavities seen in some light microscope preparations being filled with a fluid lipid rather than an aqueous solution.The lemnisci are similar in appearance to the cortex except for regions of concentrated endoplasmic reticulum.The mitochondria in the cortex of the acanthella appear to be aerobic rather than anaerobic, as they possess many cristae and are larger than the mitochondria of the adults.My thanks are due to Dr D. W. T. Crompton for advice and encouragement, and to Dr D. H. Northcote and Dr F. B. P. Wooding for time on the Phillips Electron Microscope 100. This work was carried out during the tenure of an S.R.C. studentship.