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.
The molluscs or mollusks /ËmÉlÉsks/ compose the large phylum of invertebrate animals known as the Mollusca. Around 85,000 extant species of molluscs are recognized. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms.Numerous molluscs also live in freshwater and terrestrial habitats.They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10 taxonomic classes, of which two are entirely extinct.he brain, in species that have one, encircles the esophagus. Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur.