Observations on the ultrastructure of the anterior adhesive areas and other anterior gland cells in the monogenean Merizocotyle australensis (Monocotylidae) from the nasal fossae of Himantura fai (Dasyatididae).

The anterior adhesive areas of a monocotylid monogenean, Merizocotyle australensis (Merizocotylinae), were investigated. They comprise 6 ventral apertures in 2 groups of 3 arranged at the anterolateral margins of the head. These regions are also well supplied with groups of cilia. Each aperture is 13.8 to 15.8 microm wide and contains multiple tubular projections that are covered with microvilli through which open 2 types of secretory cell ducts that carry either rod-shaped or spherical secretory bodies. The gland cell bodies that produce these 2 types of secretions co-occur at the anterior end. The 2 types of secretory bodies occur adjacent to one another and both are present in the extruded adhesive. The membranes of rod-shaped bodies are retained in the extruded glue. Rod-shaped bodies are 390 +/- 18 nm wide, at least 10.9 microm long, and show 2 types of internal periodic banding: 10.6 nm and 143 +/- 3 nm. The spherical vesicles are 130 +/- 6 nm in diameter and are electron-dense. A third secretion is present in separate ducts that also open anteriorly but emerge through the tegument between the ventral apertures. This secretion does not appear to be part of the adhesive secretion. The bodies of the third secretion are elongate, electron-dense, and 374 +/- 23 nm long. Inside the "lip" of the aperture, general body tegument abuts tegument specific to the aperture. The general body tegument is thicker, contains electron-dense vesicles, and has a ridged surface devoid of microvilli. Where the 2 kinds of tegument meet, they are connected by septate desmosomes.