Z250 - Lec14 Outline. Platyhelminthes II (2017)

Zoology 250 (2017)

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 (if present), protonephridia) & derived (anterior mouth (if present), neoophoran reproductive system) Turbellaria
2) Differ from Turbellaria in three ways (lack sense organs, epidermis lacks cilia, have a weird syncytial epithelium with microvilli)
3) Have remarkable adaptations to an endoparasitic lifestyle:
- a) adults produce massive numbers of embryos
- 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) Cl. TREMATODA (digenic flukes); typically >2 hosts; ~12,000 spp.
- a) definitive host typically a vertebrate; includes serious human parasites (liver flukes, schistosomes); live in liver or circulatory system
- b) oral (and often ventral) sucker, bulbous pharynx, 2-branch gut, no anus
- c) Chinese liver fluke has 2 intermediate hosts: i) snail (extensive asexual amplification) and ii) fish or arthropod (no reproduction), and many larval stages (miracidium, sporocyst, redia, cercaria)
5) Cl. CESTOIDEA (tapeworms); typically >2 hosts; ~6,000 spp.
- a) common gut parasites of all vertebrate classes
- b) distinctive head end (scolex); no mouth and no gut
- b) produce many segments (proglottids) packed with infective larvae
- c) encyst in intermediate host (normally no asexual amplification)

Zoology 250 (2004)

Phylum NEMERTEA

(Nemertean study image;
click on tree to see full cladogram)

6) NEMERTEA (ribbon worms; 1,200 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 (fragmentation) & regeneration, spiral cleavage
- c) differ from flatworms: body circular in cross-section, complete gut; unique blood-vascular system lined with endothelium; a spectacular proboscis resides in a rhynchocoel (a true coelom); sexes separate

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