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• 1) Flatworms, flukes, tapeworms; a well defined phylum of >15,000 free-living & parasitic species; marine, freshwater & terrestrial

  • a) considered the most primitive taxon within the Protostomia

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

  • c) lack circulatory system or coelom (body filled with parenchyma)

  • d) blind gut, mouth primitively midventral, anterior in derived taxa

  • e) trend in gut form: absent->complex->tripartite->1/2 part->absent

  • f) depend on a network of protonephridia for excretion

  • g) simple ladder-like nervous system with anterior brain

  • h) great capacity for regeneration & asexual reproduction

  • i) mostly hermaphroditic, internal fertilization & direct development; advanced forms (neoopheran) have peculiar yolk-covered eggs

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

  • a) extensively ciliated, glandular epidermis (mucous, rhabdites)

  • b) change shape with circular, longitudinal & dorsoventral muscles

  • c) 3 forms of pharynx: simple, plicate (highly eversible), bulbous

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

  • e) gonads may be absent (acoels), few (most), or many (polyclads)
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• 3) Parasitic classes all have a syncytial epithelium lacking cilia

• 4) Class Trematoda- the digenic flukes (require 2 hosts; 11,000 spp); includes some serious human parasites (liver flukes, schistosomes)

  • a) have oral (and often ventral) sucker, bulbous pharynx

  • b) complex life cycle with 1 or more intermediate hosts

• 5) Class Cestoidea- tapeworms (>2 hosts; 3,500 spp); gut parasites of vertebrates, distinctive head end (scolex, neck), lack a gut but produce multiple segments (proglottids) packed with gametes

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  Phylum NEMERTEA

• 6) free-living, marine ribbon worms, can reach quite large size; coelomate with blood-vascular system; spectacular proboscis with rhynchocoel

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