Ultrastructural characteristics of the vaginae of the basal monogenean Chimaericola leptogaster (Leuckart, 1830).

In the first study of the vaginal ultrastructure of any monogenean, the paired vaginae of Chimaericola leptogaster, a basal polyopisthocotylean from the gills of a holocephalan, are described. Each vagina opens laterally. A unique feature of this relict parasite is a short vagino-intestinal connecting duct, the lining of which is separated by septate junctions from the linings of the vagina and the intestine. After giving rise to this vagino-intestinal duct, each vagina travels in a posterior direction and opens into the vitelline collecting duct. The lining of each vagina close to the vaginal pore resembles the body tegument, the syncytial lining of which measures about 2.5 μm in thickness and has a dense surface layer 0.2 μm in thickness and different apical projections. Proximal to this and before the entrance to the vaginal-intestinal duct, the vaginal lining is characterised by: uniformly distributed electron-dense, lanceolate, spine-like surface structures of about 1.7 μm in length; three kinds of surrounding glands with three kinds of secretory granules (abundant oval or rounded electron-dense granules 0.7-2.5 μm in length, much less numerous spheroidal, vesicular granules 0.3-0.4 μm in diameter, and tightly packed, spheroidal granules 0.8 μm in diameter and containing fine particulate material of low density); and four kinds of sensory receptors (three uniciliate and one aciliate). These features are absent from the rest of the vagina. The likely roles of the vagino-intestinal connection, the different gland cells, sensory structures, and the armament of the distal vaginal regions are discussed. Considering the suggested polyopisthocotylean ancestor for the Neodermata, the relict parasite fauna of holocephalans and the unique vagino-intestinal connection in C. leptogaster, which is also known in turbellarians, an ancient origin for chimaericolids is supported.