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) 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)

  • b) 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) lack circulatory system or coelom (body is filled with a 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; advanced forms (neoopheran) have peculiar yolk-covered eggs

• 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 w/ 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

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

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

Back to Zool 250 Home Page