PHYLUM PLATYHELMINTHES

Class Turbellaria: predominantly free-living flatworms. Ventral mouth leads to stomodeal pharynx.

• Order Nemetodermatida: Pharynx (when present) is simple. Gut cavity has interdigitating processes from the intestinal lining.
• Order Acoela: Pharynx (when present) is simple. No permanent gut cavity: mouth leads to solid or cellular endodermal mass.
• Order Catenulida: Elongate marine and freshwater forms. Simple pharynx, simple saclike gut. Mesenchyme often reduced to a fluid matrix.
• Order Macrostomida: simple pharynx. Gut simple, saclike.
• Order Lecithoepitheliata
• Order Rhabdocoela: bulbous pharynx. Saclike gut without diverticula.
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  • Suborder Dalyellioida: anterior mouth
  • Suborder Typhloplanoida
  • Suborder Kalyptorhynchia: complex eversible proboscis at anterior end not associated with mouth or pharynx.
  • Suborder Temnocephalida: small symbionts with posterior sucker or anterior tentacle used for attachment.
• Order Prolecithophora: Pharynx plicate or bulbous. Simple gut.
• Order Proseriata
• Order Tricladida: Cylindrical plicate pharynx. Gut three-branched with numerous diverticula. Includes the familiar planarians.
• Order Polycladida: Gut many-branched with diverticula.

Class Monogenea: Monogenetic flukes. Parasitic. Most are ectoparasites. Life cycle includes only one host.

• Subclass Monopisthocotylea: Ophisthaptor (rear sucker) simple and single.
• Subclass Polyopisthocotlea: Ophisthaptor complex, with multiple suckers.

Class Trematoda: Flukes. Parasitic. Most are endoparasitic. Most have 2 or 3 hosts in life cycle.

• Subclass Digenea: 2-3 hosts in life cycle. First intermediate host is mollusc, final host vertebrate. Oral and usually ventral sucker.
• Subclass Aspidogastrea: 1 or 2 hosts. Single or intermediate host is a mollusc. Final host when present is fish or turtle. No oral sucker, large ventral sucker.

Class Cestoda: tapeworms. Exclusively endoparasitic.

• Subclass Cestodaria: small group of flattened tapeworms lacking scolex and proglottids. Endoparasites in cartilagenous and primative bony fish, occasionally in turtles.
• Subclass Eucestoda: "true" tapeworms. Body consists of scolex with hooks and/or suckers, followed by series of proglottids. Can be quite large (up to 10 meters long). All endoparasites of vertebrates. Most require one or more intermediate hosts in life cycle.

Body Plan	Triploblastic (three germinal layers: endoderm, mesoderm, ectoderm). Acoelomate: space between body wall and gut is occupied by mesenchyme which may be densely packed with cells or syncytial and fibrous networks with fluid-filled spaces. Bilateral symmetry, with distinct anteroposterior differentiation, though "head" is ill-defined. Generally flat in shape (smaller members are more round). Of the four classes, three are wholly parasitic, and only one (Turbellaria) has mostly free-living members.
Locomotion	In free-living turbellarians, epidermal cells are ciliated. Larger ones may also use body musculature to undulate, allowing them to creep or swim. Flukes and cestodes are parasitic and mostly nonmotile, though they may use suckers to hold or move in or on the host.
Feeding and Digestion	Feeding in turbellarians is mostly using an eversable pharnyx located on the ventral midbody surface. Most turbellarians are carnivorous, and usually enfold prey and then absorb with pharynx. Some secrete poisons. The gut has no anus. Digestion is extracellular. Gut extends throughout the body. Transport is by diffusion aided by body movement. Flukes also feed through mouth (located anteriorly), but lack an eversible pharynx. Cestodes lack any mouth, and absorb nutrients through the epidermal layer.
Sense Organs	Most developed in turbellarians. These have photoreceptors (ocelli), chemoreceptors, mechanoreceptors, and statocysts. Ocelli are usually paired and directional (using pigment cups). Flukes and cestodes have fewer sense organs, usually just mechanoreceptors and chemoreceptors.
Nervous System	"Ladderlike" nervous system of two lateral nerve trunks connected by transverse commisures. In turbellarians, varies from a simple nerve net to distinctly bilateral arrangement with ganglia in "head". Many show tendancy to specialize neurons into sensory and motor, with one-way conduction of impulses. Fluke cerebral ganglia show two distinct lateral lobes with a dorsal transverse commisure. Cestodes have a complex nerve ring in the scolex ("head"),
Respiration and Gas Exchange	All exchange is on a cellular level, by diffusion, which is why flatworms are flat. Nitrogenous waste mostly as ammonia, lost through diffusion. Endoparasitic forms often rely on anaerobic metabolism, with some unusual end products (e.g., lactate, succinate, alanine, long-chain fatty acids).
Circulation	No specialized circulatory system.
Osmoregulation	By protonephridia. Use flame cells at internal ends of complex and branching network of collecting ducts which terminate at nephridiopores. Protonephridia may be used in cestodes additionally for removal of anaerobic metabolism endproducts.
Reproduction	Asexual reproduction common in turbellarians. Usually by transverse fission, occasionally by fragmentation. Sexual reproduction in turbellarians is hermaphroditic, but fertilisation usually by cross-mating. Digenic flukes and cestodes have several stages involving different hosts (asexual reproduction in the stages may be used to increase greatly the number of individuals from a single egg). Cestodes cross-fertilize when mates available, otherwise self-fertilize.