Zoology 250 (2002) Phylum PLATYHELMINTHES: Flatworms ( Platyhelminthes study images; click on tree to see full cladogram)

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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, & tapeworms. It is a well-defined phylum of >15,000 free-living & parasitic species found in marine, freshwater & terrestrial habitats.

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

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

  • c) have a blind gut (no anus); the mouth is primitively mid-ventral but becomes anterior in derived taxa

  • c) generally small in size or considerably flattened in form

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

  • e) depend on a network of protonephridia for excretion

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

  • g) tremendous capacity for regeneration & asexual reproduction

  • h) most are hermaphroditic, have internal fertilization & direct development; primitive forms (archoopheran) show spiral cleavage; advanced forms (neoopheran) have peculiar yolk-covered eggs; larval form (if present) not a trochophore

  • i) classification is a problem: Turbellaria are paraphyletic

• 2) Class TURBELLARIA- mostly free-living flatworms (3,000 spp.)

  • a) have an extensively ciliated, glandular epidermis (multiciliated cells, produces mucous & crystalline rhabdites)

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

  • c) gut may be absent (acoels), complex (polyclads), tripartite (triclads), or unbranched (rhabdocoels); mouth mainly midventral

  • d) 3 forms of pharynx: simple, plicate (highly eversible), bulbous; ingest food via different techniques

  • e) gonads may be absent (acoels), few (most), or many (polyclads)

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

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

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