Ultrastructural studies of Austramphilina elongata (Cestoda, Amphilinidea)

Abstract

The fine structure of larval Austramphilina elongata is described using serial semithin and ultrathin sections. Densely packed germ cells with many ribosomes and mitochondria and with large Golgi complexes fill the middle third of the body. Some necrotic nuclei were observed near the anterior end. The neodermis consists of a subepidermal syncytium connected to pericarya in the parenchyma by means of cytoplasmic processes containing peripheral microtubules; electron-dense ovoid bodies condense in these processes. Myoblasts are connected to muscle fibres by means of cytoplasmic connections rich in mitochondria. Twelve (exceptionally eleven) type I gland cells containing large secretory granules and extensive granular endoplasmic reticulum are located in the dorso-posterior part of the body; they open through 12 (or 11) discrete ducts into an anterior invagination of the tegument which is covered by epidermis and not connected to the outside. Ten type II gland cells containing elongate secretory granules with regularly arranged longitudinal microtubules are located ventral to the type I cell bodies; they open on a ventral papilla a short distance behind the anterior end. Ten type III gland cells containing irregularly round-oval secretory granules with coiled microtubules are located anterior and ventral to the type I gland cells; they open through five discrete ventro-anterior openings on each side of the body. Ducts of all gland cells have mitochondria and microtubules. The spermatozoon has a basic pattern of two axonemes, each with a single central filament, a mitochondrion (mitochondria), and a row of surface microtubules interrupted by the axonemes. In the tips of epidermal cilia, doublet 1 and doublets adjacent to it lose their microtubules B first and close in on the central pair of filaments in a spiral fashion, enclosing an electron-dense rod. Presence of a neodermis and ultrastructure of the spermatozoon support the validity of the taxa Neodermata Ehlers and Trepaxonemata Ehlers and are strong evidence against a phylogenetic relationship of the ‘cestodarians’ — cestodes with the Acoelomorpha; this is also indicated by the ultrastructure of sense receptors and epidermal ciliary rootlets.