Ultrastructural observations on the marginal hooklets of the monogenean gill parasite Cichlidogyrus halli typicus.

The marginal hooklets of adult Cichlidogyrus halli typicus have the same morphology, each being composed of a handle, a curved blade with a tapering guard and an accessory domus. In addition, all the hooklets except those of pair II have a long extension attached to the proximal end of the handle. Ultrastructural observations reveal that the handle consists of three distinct layers whereas both the guard and the proximal region of the blade lack the outer electron-dense layer. The dense core is absent from the rest of the blade. The extension has a fibrous structure and an affinity for eosin, unlike the rest of the hooklet. There is a conspicuous cavity associated with each hooklet. The cavity is partly lined with a syncytial cytoplasmic epithelium containing secretory bodies. The lining is completed by the cytoplasmic covering of the handle; this covering may be the surviving oncoblast cell and has knob-like structures of unknown function projecting into the cavity. The domus has the appearance of two filaments with the light microscope but with the electron microscope these are seen to be the thickened edges of a single gutter-shaped sclerite.