A Contribution Towards Understanding of ‘The Primitive Fucoid’

S ummary The spermatozoids of three genera of Cystoseiraceae ( Halidrys, Cystoseira and Bifurcaria ) have been investigated electron microscopically with respect to their external morphology, and two of them ( Cystoseira and Halidrys ) also with respect to their microanatomy for the purpose of comparison with Fucus on the one hand and a brown algal zoospore on the other. Special attention has been devoted to the flagellar ‘root’ which is shown to be the homologue of the proboscis of Fucus and to be present also in the zoospore of Scytosiphon . Comparison between Scytosiphon, Dictyota and the Fucoids suggests that the true function of the flagellar root is not mechanical but metabolic or conductional, perhaps in some relation to respiration or gaseous exchange. The evidence is interpreted as giving strong support to the view that Fucus is not primitive but highly specialized and that Cystoseira has remained closer to an ancestral condition in a number of different characters.