The endoparasitic helminth communities of the European eel, Anguilla anguilla (L.), were investigated in four meanders, cut off from the rivers Leie and Scheldt in western Handers, Belgium. Six species of helminths (2 cestodes, 2 nematodes and 2 acanthocephalans) were found. The dominant parasite species was the nematode Anguillicola crassus (Kuwahara, Niimi et Itagaki, 1974) infecting 79% of the eel population with intensities up to 112 specimens per fish. At two localities no acanthocephalans could be found, whereas these parasites were very common at the other sites. The prevalence, mean intensity, intensity and abundance, their correlation to the body length, and the frequency distributions were analysed. The site selection of parasites is in relation to food composition and feeding habits of eels, physiological and structural differences in the intestine and possible interspecific competition were discussed.
Structural differences of microtriches and distal cytoplasm of the tegument in the apical and lateral suckers, scolex proper and neck of Proteocephalus macrocephalus (Creplin,1815) were studied. The microthrix border in the apical sucker is formed by filamentous microtriches only. The frontal and lateral parts of scolex bear mainly filamentous microtriches, but seldom short conoid types occur. The transitional zone between scolex and neck is covered mainly with short conoid microtriches. The neck bears blade-like microtriches. In the apical and lateral suckers, the basal plasma membrane of the distal cytoplasm adheres to the basal lamina at some points only, forming thus a lacunal system at the base of the sucker tegument. In the scolex proper and neck region, the basal plasma membrane of the tegument is connected continuously with the basal lamina. The distal cytoplasm is penetrated by two types of gland cell ducts and cibate sensory receptors. Possible functions of different parts of the microthrix border are discussed.