Ultrastructural observations on rhabdite formation in the planarian, Artioposthia triangulata

The epidermis of the predatory terrestrial flatworm, Artioposthia triangulata has been examined by transmission electron microscopy for the presence of rhabdiform secretions. Two types of secretion are present: epidermal rhabdoids, produced by a special type of epidermal cell and true adenal rhabdites produced by gland cells beneath the epidermis. The epidermal rhabdoids are formed from Golgi-derived vesicles, which fuse together to form the developing rhabdoid. Within the latter is a filamentous network on which granular material is deposited and coalesces to form a rod-shaped inclusion. The rhabdoids accumulate in the apical region of the cell and release their contents from the apical surface. The adenal rhabdites are formed by Golgi-derived vesicles, which become more elongated and their contents more electron-dense as they mature. The vesicles fuse together to form the primordial rhabdite, which continues to lengthen with the addition of further vesicles. The neck of the rhabdite-forming cell passes between the muscle layers and through the basement membrane to open into the base of the epidermal cell. The rhabdites move from the cell body through the neck into the cytoplasm of the epidermal cell and make their way to the apical surface where they are released to the exterior.