The structure of the zoospore cysts of various members of the Saprolegniaceae has been studied by electron microscopy. In Saprolegnia ferax, S. dioica, in Isoachlya eccentrica and I. unispora the primary cysts were smooth, whilst the secondary cysts bore stalked double-headed hooks. In S. parasitica the secondary cysts bore tufts of longer hooks. In Protoachlya, Achlya, and Brevilegnia neither type of cyst bore hooks. In Dictyuchus sterile the secondary cysts bore large spiny projections. The primary cysts in many species bore tufts of radiating hairs, and it is suggested that these are the remains of ciliated flagella.
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Meier et al. (1954) studied this question.
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