The structure of the extracellular matrix (ECM) of representatives of all four sections of the genus Volvox was examined by a combination of light- and electron-microscopic methods. On the basis of these observations, plus published descriptions of aspects of ECM organization in other members of the order Volvocales, a system of nomenclature is proposed, to facilitate discussion of comparative morphology and phylogeny of the ECM in the order. In this system the ECM is divided into four main zones: the flagellar zone (FZ), which consists of attachments to and specializations of the ECM around the flagella; the boundary zone (BZ), which consists of portions of the ECM that (except in periflagellar regions) are continuous over the surface of the organism and are not structurally continuous with deeper layers; the cellular zone (CZ), which consists of specializations, other than those of the FZ, around individual cells; and the deep zone (DZ), which consists of components that fill the central region of the organism, internal to CZ. An empirically based set of hierarchical subdivisions of these zones is then proposed that permits specific identification of most morphologically distinct ECM components. The fact that not all zones and subzones are present in all members of the order means that this system permits identification of those ECM structures that have been gained or lost during Volvocalean evolution. Species-specific differences in the structure of virtually all aspects of the ECM were seen among the Volvox species examined in this study. However, the fact that such differences cannot always be used as diagnostic characters for the four divisions of the genus was demonstrated by the observation that in certain ECM features two members of the same division (V. carteri f. nagariensis and V. carteri f. weismannia) differ markedly in structure from one another, with one member of the pair resembling a member of another division. Thus many details of ECM organization appear to be under separate control, and capable of independent evolution.

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