الفهرس 
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Abstract
The present study deals with the relationship between parasites and their hosts in some gastropods collected from the River Nile at Sohag Province. A number of snails were collexted from the main study location including: Cleopatra bulimoides, Lanistes carinatus, Melania tuberculata, Bellamya unicolor and Biomphalaria Alexandrina. The abundance and distribution as well as the infection percentage in these snails were studies.
Twelve types of cercariae were collected from the investigated snails, belonging to 4 main groups.
The intertubular spaces of the infected digestive gland of Cleopatra bulimoides, Lanistes carinatus, were packed with large number of sporocycts and developing rediae. The larval stges in the two hosts caused a degree of tissue damage for the hosts and defence reaction from these snails against the parasites.
The miracidial host-finding behavior consists of two mechanisms which lead to the accumulation of the miracidia around the host active space, an increase in the rate ofg change of direction ( RCD ) when entering the active space and turn back ( TB ) when leaving it. The chemical and enzymatic analysis of snail conditioned water from the snail Biomphalaria Alexandrina and B. glabrata showed that it consists of six fractions. It was found that the Egyptian strain miracidia attracted to the fraction IV of Biomphalaria Alexandrina SCW,