An account of the structure of plants naturally begins with the cell which is the proximate unit of organic structure. The elementary unit of plant structure, as of animal structure, is the cell. Faunistically, although not geographically, the Nearctic and Palaearctic areas must form the two subdivisions of one great unit, for which the  Holarctic region  is now the generally accepted term.
Definition: a subclass considered a phylum of the kingdom Protista in some classifications; includes holozoic or saprozoic flagellates.
Mollusks are members of the phylum mollusca. Mollusks have been evolving in the sea for over 600 million years and there are more than 100 000 mollusks species. The mollusks phylum includes tiny snails, squid, octopus, clams, oysters, abalone, and cuttlefish.They are considered to be the most intelligent of the invertabrate species.he study of mollusks is called malacology. There are four main classes in the phylum mollusca, gastropods, bivalve, cephalopod and Polyplacophora. Gastropods include nudibranches, slugs and snails. They compose over one third of the mollusk group and most are single spiral shells but some have no shells.