Platyhelminthes

Abstract

The phylum Platyhelminthes comprises dorso-ventrally flattened worms commonly known as flatworms (from the Greek platys, meaning flat, and helminthos, meaning worm) (for a general overview of this phylum, see Hyman 1–951; Rieger et al. 1–991). Platyhelminthes are one of the largest animal phyla after arthropods, mollusks, and chordates and includes more than 2–0,0–00 species, more than half of which are parasitic flatworms. Free–013;living flatworms (classically referred to as –018;Turbellaria–019;) live in a large variety of habitats, from freshwater springs, rivers, lakes, and ponds to the ocean and moist terrestrial habitats. Their size ranges from microscopic worms to the 3–0 m long tapeworms found in the sperm whale. Free–013;living flatworms are most often white, brown, grey, or black; polyclads (marine flatworms) and terrestrial species usually display bright colours and patterns. Molecular phylogenetic studies place the Platyhelminthes within the Spiralia (=Lophotrochozoa) clade. The most recent internal phylogenies support the subdivision of the Platyhelminthes into two main groups–03A; the earliest branching lineages grouped into the paraphyletic –018;Archoophora–019; and the more divergent monophyletic Neoophora (Laumer and Giribet 2–014; Riutort et al. 2–012). The –018;Archoophora–019; includes those groups with endolecithal eggs. They are exclusively free–013;living organisms and are classified into three orders: Catenulida, Polycladida, and Macrostomida (Fig. 3.1). The Neoophora includes all groups with ectolecithal eggs. It comprises several free–013;living orders, together with the parasitic groups (the classes Trematoda, Cestoda, and Monogenea) united under the monophyletic Neodermata.