Z250 - Lec13 Outline. Platyhelminthes II (2002)

Zoology 250 (2002)

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 general (dorsoventrally flattened body, blind gut, protonephridia, hermaphroditic) & 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 promote dispersal; extensive asexual reproduction in some can greatly amplify total fecundity
4) Class TREMATODA- (digenic flukes); require 2 hosts; 11,000 spp
- a) definitive host nearly always a vertebrate; includes some serious human parasites (liver flukes, schistosomes)
- b) oral (& often ventral) sucker, bulbous pharynx, paired intestines
- c) Chinese liver fluke has 2 intermediate hosts: i) snail (extensive asexual amplification) and ii) fish or arthropod (no reproduction)
5) Class CESTOIDEA- tapeworms (>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 (1,000 spp); predatory, mostly marine ribbon worms
- 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 & 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)