Scanning electron microscopy of Polystoma integerrimum (Monogenea, Polystomatidae).

The tegumental surfaces of both the bladder (normal) adult and branchial (neotenic) adult of the dimorphic monogenean Polystoma integerrimum are elevated into ridges forming a honeycomb-like pattern of hexagons. In some regions, upright microvilli are located at the angles of the hexagons. The two hamuli of the adult forms are sheathed with the ornamented tegument except for the naked distal spine. Larvae with haptors at the two-sucker stage of development have a tegument with the elevated honeycomb design characteristic of the adults. Developing hamuli appear as fully sheathed, conical elevations. The 16 marginal hooklets of the larval haptor are covered by tegument, with the exception of their strongly curved spines. Genetic blueprints for normal and neotenic development, contained in the genome of all larvae hatching from the eggs of normal individuals, appear to diverge only in relation to the different rates of embryonic development of the two adult forms and the incomplete ontogeny of the neotenic adult.