There is no unanimity concerning the classification of platyhelminths. The following classification should be considered provisional.

• Phylum Platyhelminthes (flatworms)

  Flat, unsegmented worms; gastrovascular cavity and respiratory, skeletal, and circulatory systems absent; excretion by means of flame-bulb protonephridia; mesenchyme fills all spaces between organ systems; a variable number of longitudinal nerve cords with transverse connectives; body structure triploblastic (i.e., 3 embryonic layers); reproductive system hermaphroditic and complex.

  • Class Turbellaria

    Epidermis usually ciliated at least in part, provided with rhabdoids (minute rodlike structures); body unsegmented; gut present except in order Acoela; life cycle simple; mostly free-living, some ectocommensal, endocommensal (i.e., living, respectively, outside or inside another organism without harming it), or parasitic; about 3,000 species.

    • • • Order Acoela

          Exclusively marine; mouth present; pharynx simple or lacking; no intestine; without protonephridia, oviducts, yolk glands, or definitely delimited gonads; about 200 species.

        • Order Neorhabdocoela

          Saclike linear intestine; protonephridia and oviducts usually present; gonads few, mostly compact; nervous system generally with 2 longitudinal trunks; about 600 species.

        • Order Catenulida

          Mostly freshwater; about 70 species.

        • Order Macrostomida

          Mostly inhabiting the areas between grains of sand; about 200 species.

        • Order Polycladida

          Pharynx simple, bulbose, or plicate (many ridges); intestine may have short diverticula, or pockets; protonephridia paired; testes usually numerous; penis papilla generally present; nervous system with 3–4 trunks; nearly 800 species.

  • Class Monogenea

    Oral sucker lacking or weakly developed; posterior end with large posterior adhesive disk (opisthaptor) usually provided with hooks; excretory pores paired, anterior and dorsal; parasites of the skin and other superficial locations, especially on the gills of fish; life cycle simple, no alternation of hosts; about 1,100 species.

  • Class Cestoda (tapeworms)

    Elongated endoparasites with alimentary canal lacking; epidermis modified for absorption and secretion; usually divided into segments (proglottids); adhesive organs limited to anterior end; except in Cestodaria, adult stages almost entirely parasites of vertebrates; life cycles complicated with 1 or more intermediate hosts; about 3,500 species.

    • Subclass Cestodaria

      Unsegmented tapeworms containing 1 set of genitalia; parasites of the body cavity or intestine of annelid worms or fish; about 105 species.

      • • Order Amphilinidea

          Uterus long and N-shaped; genital pores at or near posterior extremity; intestinal parasites of teleosts (bony fish); 105 species.

        • Order Caryophyllidea

          Uterus a coiled tube; genital pore well separated from posterior extremity; intestinal parasites of teleosts, occasionally in annelids; about 85 species.

        • Order Gyrocotylidea

          Testes confined to anterior region; genital pores near anterior end; parasitic in intestine of fish of the genus Chimaera; 105 species.

    • Subclass Eucestoda

      Polyzoic tapeworms with scolex (head) of varying structure; body usually with distinct external segmentation; parasitic in intestine of vertebrates. Known commonly as the “true” tapeworms; well more than 3,000 species.

      • • Order Tetraphyllidea

          Scolex with 4 bothridia (leaflike muscular structure); vitellaria located in lateral margins of proglottids; genital pores lateral; parasites of elasmobranchs; about 200 species.

        • Order Lecanicephalidea

          Reproductive system similar to Tetraphyllidea, but scolex divided into an upper disklike or globular part and a lower collarlike part bearing 4 suckers; mainly parasites of elasmobranchs; 5 species.

        • Order Proteocephalidea

          Scolex with 4 suckers, sometimes a 5th terminal one; vitellaria located in lateral margins; genital pores lateral; mainly parasites of cold-blooded vertebrates; about 185 species.

        • Order Diphyllidea

          Two bothridia, each sometimes bisected by a median longitudinal ridge; large rostellum (cone-shaped or cylindrical projection) armed with dorsal and ventral groups of large hooks; cephalic peduncle (fleshy stalk on head) with longitudinal rows of T-shaped hooks; genital pore median, parasitic in elasmobranchs; 1 genus, Echinobothrium, with 2 species.

        • Order Trypanorhyncha

          Scolex with 2 or 4 bothridia; vitellaria in continuous sleevelike distribution; parasites of elasmobranchs; about 115 species.

        • Order Pseudophyllidea

          Scolex with 2 elongated, shallow bothria, 1 dorsal and 1 ventral; genital pore lateral or median. Vitellaria lateral or extending across proglottid and encircling other organs; parasites of teleosts and land vertebrates. Order includes the largest of all known tapeworms, Polygonoporus giganticus, which reaches lengths of 30 metres (100 feet) in sperm whales. About 315 species.

        • Order Nippotaeniidea

          Scolex bears 1 apical sucker; parasites of freshwater fish; 1 genus, Nippotaenia; 3 species.

        • Order Cyclophyllidea (Taenoidea)

          Scolex with 4 suckers; no uterine pores; 1 compact vitellarium behind ovary; mainly parasites of birds and mammals; probably more than 2,000 species.

        • Order Aporidea

          No sex ducts or genital openings; parasites of swans, ducks, and geese; 4 species.

        • Order Spathebothriidea

          Scolex without true bothria or suckers; strobila with internal segmentation but no external segmentation; parasites of marine teleosts; 10 species.

  • Class Trematoda (flukes)

    Ectoparasites or endoparasites; no ciliated epidermis; body undivided; adhesive organs well-developed; life cycles generally complex with 2 or more hosts; about 11,000 species.

    • Subclass Aspidogastrea

      Oral sucker absent; main adhesive organ occupying almost the entire ventral surface, consists of suckerlets arranged in rows; excretory pore single and posterior; endoparasites of vertebrates, mollusks, and crustaceans; about 35 species.

    • Subclass Digenea

      Oral and ventral suckers generally well-developed; development involves at least 1 intermediate host; usually endoparasites of vertebrates; about 9,000 species.

      • • Order Strigeidida

          Cercaria (immature form) fork-tailed; penetration glands present; 1–2 pairs of protonephridia; about 1,350 species.

        • Order Echinostomida

          Cercaria with simple tail and many cyst-producing glands; life cycle with 3 hosts; about 1,360 species.

      • • Order Plagiorchida

          Cercaria typically armed with a stylet; encystment in invertebrates, rarely vertebrates; excretory vessels not open to the exterior. Most representatives require 3 hosts to complete one life cycle. Many hundreds of species.

        • Order Opisthorchiida

          Cercaria never armed; excretory pores open on margins of tail; about 700 species.

Critical appraisal

There is disagreement on many aspects of the taxonomy of Platyhelminthes, especially regarding class divisions. For example, some authorities consider Monogenea to be a subclass within the class Trematoda.