Z250 - Lec14 Outline. Platyhelminthes II (2005)

Zoology 250 (2005)

Phylum PLATYHELMINTHES II:
Parasitic taxa

( Platyhelminthes study images;
click on tree to see full cladogram)

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1) Parasitic classes share many traits with generalized (dorsoventrally flattened body, blind gut, protonephridia, hermaphroditic) and derived (anterior mouth, neopheran reproductive system) Turbellaria
2) Differ from Turbellaria in three ways (lack sense organs, epidermis lacks cilia, have a weird syncytial epithelium with microvilli)
3) They have remarkable adaptations to a parasitic lifestyle:
- a) adults produce massive numbers of gametes
- b) most have complex life cycles (two or more hosts)
  - final= definitive host is the site of sexual reproduction
  - one or more intermediate hosts: main function is to enhance transmission to definitive host; may include tremendous amplification of infective stages via asexual reproduction
4) Class TREMATODA (digenic flukes); require at least 2 hosts; 11,000 spp.
- a) definitive host nearly always a vertebrate; includes some serious human parasites (liver flukes, schistosomes)
- b) oral (and often ventral) sucker, bulbous pharynx, gut has two branches
- c) Chinese liver fluke has 2 intermediate hosts: i) snail (extensive asexual amplification) and ii) fish or arthropod (no reproduction)
5) Class CESTOIDEA (tapeworms); require at least 2 hosts; 3,500 spp.
- a) common gut parasites of all vertebrate classes
- b) distinctive head end (scolex, neck); no mouth or gut
- b) produce many segments (proglottids) packed with infective larvae
- c) encyst in intermediate host (enhances ingestion by definitive host)

Phylum NEMERTEA

(click on tree to see full cladogram)

6) NEMERTEA (ribbon worms; 1,000 spp); predatory, mostly marine
- a) can reach quite large size (10-15 cm common; some to 30 m!)
- b) share many traits with flatworms: multiciliated glandular epidermis, protonephridia, ladder-like nervous system, capable of asexual reproduction and regeneration, spiral cleavage
- c) differ from flatworms: body is circular in cross-section, complete gut; unique blood-vascular system lined with endothelium; spectacular proboscis with rhynchocoel (a true coelom)