MATERIAL AND METHODS
This work was carried out in the Department of Zoology, Sheffield University, the roach and gudgeon being supplied by the Sheffield Corporation Water Works from their ponds surrounding the city, while the mirror carp came from the Surrey Trout Farm (Haslemere). The Red Sea types used for comparison were collected personally from the neighbourhood of the Marine Biological Station at Ghardaqa, while other supplementary types came from various English waters.