Zoology 250 (2007) Phylum PLATYHELMINTHES (I): Flatworms (Platyhelminthes study images; click on tree to see full cladogram; web links to other sites)

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Worm-like forms occur in 12 different phyla! "Worms" are simply soft-bodied animals that are longer than wide.

• 1) Ph. PLATYHELMINTHES includes the flatworms, flukes, and tapeworms. It is a well-defined phylum of >20,000 free-living and parasitic species found in marine, freshwater and terrestrial habitats.

  • a) generally small in size (<2-3 cm) or flattened in form; can reach 60 cm (!) - larger worms are relatively flatter

  • b) have a blind gut: mouth but no anus

  • c) no circulatory system or coelom (filled with spongy parenchyma)

  • d) depend on a network of protonephridia for excretion

  • e) have a simple ladder-like nervous system with anterior brain

  • f) tremendous capacity for regeneration and asexual reproduction

  • g) most are hermaphroditic, have internal fertilization and direct development; primitive forms (archoopheran) show spiral cleavage; advanced forms (neoopheran) have peculiar yolk-covered eggs; larval form (if present) not a trochophore: Muller's larva

  • h) classification is a problem: the Turbellaria is paraphyletic

  • i) the most primitive taxon within the Protostomia:
    • has some protostome characteristics (spiral cleavage, blastopore becomes mouth, multiciliated cells)
    • lacks others (coelom, circulatory system, anus, trochophore larvae)

• 2) Class TURBELLARIA- mostly free-living flatworms (5,000 spp.) ; composed of 9 orders but will only focus on 4

  • a) have an extensively ciliated, glandular epidermis (name= "whirlpool", multiciliated cells, produces mucous and crystalline rhabdites)

  • b) vary body shape with circular, longitudinal and dorsoventral muscles

  • c) gut may be absent (Acoela), complex (Polycladida), tripartite (Triclada), or unbranched (Neorhabdocoela)

  • d) 3 forms of pharynx: simple, plicate (highly eversible), bulbous (pumping); ingest food via different techniques; mouth is mid-ventral but becomes anterior in derived rhabdocoels

  • e) gonads may be absent (acoels), few (most), or many (polyclads); those with gonads are hermaphroditic; some have bizarre copulation

  • f) recent evidence suggests acoels may be the sister taxon to the Bilateria!

Links to other sites

For some breathtaking images of the different forms and stunning colors that turbellarian flatworms exhibit see Marine Flatworms of the World.

View a remarkable video sequence of elaborate mating behavior in polyclad flatworms at Fighting to Mate: Flatworm Penis Fencing.

Check out this great site to explore the anatomy of acoel flatworms.