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Introduction
Digenetic trematodes are implicated in several important parasitic diseases of humans and animals, such as schistosomiasis, clonorchiasis, opisthorchiasis, fascioliasis and diplostomiasis. This is due to the capability of their developmental stages to adapt to completely different environments or both poikilothermic and homeothermic hosts (Solis-Soto & De Jong-Brink, 1995). Trematodes also produce neurosubstances that enable them to evade the immune activities of the hosts (Duvaux-Miret et al., 1992). Since the neurosubstances originate from the host in response to the parasitic stimulus (De Jong-Brink, 1995), and the release of neurosecretory material accords with the parasite's change from poikilothermic to homeothermic hosts (Gustafsson & Wilkgren, 1981), the study of the whole nervous system would contribute greatly to the knowledge of the phylogeny of trematodes (Grabda-Kazubska & Moczon, 1981).
The catfish, Clarias gariepinus, is the host to three Tylodelphys species metacercariae co-existing in the cranial cavities (Musiba & Nkwengulila, 2006; Chibwana & Nkwengulila, 2010). Morphologically, the three diplostomid metacercariae are easily separated. Tylodelphys mashonense has a flat, oval body with fore- and hindbody distinct, though not as distinct as in the European congeners, and well-developed pseudosuckers (Beverley-Burton, 1963). Metacercariae of two other Tylodelphys species, designated as Tylodelphys spp. 1 and 2, are morphologically similar. They have a dorsoventrally flattened forebody and a small, conical and translucent hindbody, with no clear separation between them, and the pseudosuckers are lacking (Chibwana & Nkwengulila, 2010). They can be differentiated based on size and the presence of gonadal enlargen in Tylodelphys sp. 2. Furthermore, molecular analysis of the three morphotypes revealed the presence of three clearly distinct species (Chibwana et al., 2013). Since the three Tylodelphys metacercariae types co-exist in the same locality within the fish host, it seems interesting to study whether their nervous system structures are similar with respect to the similar function they perform.
Studies of the nervous system of diplostomids are commonplace in Europe; those of Diplostomum pseudospathaceum in particular - its nervous system has been well documented in all stages of the life cycle (Niewiadomska & Moczon, 1982, 1984, 1987, 1990). In Africa, on the other hand, knowledge of the nervous system of diplostomid species is missing, with the exception of the partially studied T. mashonense (Beverley-Burton, 1963). Accordingly, the aim...