Zoology 250 Lecture 12
Phylum PLATYHELMINTHES
(Flatworms, flukes, tapeworms)
- 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) blind gut, mouth primitively midventral, anterior in derived taxa
- d) lack circulatory system or coelom (body filled with parenchyma)
- e) depend on a network of protonephridia for excretion
- f) simple ladder-like nervous system with anterior brain
- g) great capacity for regeneration & asexual reproduction
- h) most 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 (multiciliated, produces 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)
- 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) oral (& often ventral) sucker, bulbous pharynx, paired intestines
- 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 many segments (proglottids) packed with infective larvae
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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(revised Jan. 14, 1999)