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Worm-like forms occur in 12 different phyla! "Worms" are simply soft-bodied animals that are longer than wide.
- 1) Ph. PLATYHELMINTHES includes the flatworms, flukes, and tapeworms. It is a well-defined phylum of >20,000 free-living and parasitic species found in marine, freshwater and terrestrial habitats.
- a) generally small in size (<2-3 cm) or flattened in form; can reach 60 cm (!) - larger worms are relatively flatter
- b) have a blind gut: mouth but no anus
- c) no circulatory system or coelom (filled with spongy parenchyma)
- d) depend on a network of protonephridia for excretion
- e) have a simple ladder-like nervous system with anterior brain
- f) tremendous capacity for regeneration and asexual reproduction
- g) most are hermaphroditic, have internal fertilization and direct development; primitive forms (archoopheran) show spiral cleavage; advanced forms (neoopheran) have peculiar yolk-covered eggs; larval form (if present) not a trochophore: Muller's larva
- h) classification is a problem: the Turbellaria is paraphyletic
- i) the most primitive taxon within the Protostomia:
- has some protostome characteristics (spiral cleavage, blastopore becomes mouth, multiciliated cells)
- lacks others (coelom, circulatory system, anus, trochophore larvae)
- 2) Class TURBELLARIA- mostly free-living flatworms (5,000 spp.) ; composed of 9 orders but will only focus on 4
- a) have an extensively ciliated, glandular epidermis (name= "whirlpool", multiciliated cells, produces mucous and crystalline rhabdites)
- b) vary body shape with circular, longitudinal and dorsoventral muscles
- c) gut may be absent (Acoela), complex (Polycladida), tripartite (Triclada), or unbranched (Neorhabdocoela)
- d) 3 forms of pharynx: simple, plicate (highly eversible), bulbous (pumping); ingest food via different techniques; mouth is mid-ventral but becomes anterior in derived rhabdocoels
- e) gonads may be absent (acoels), few (most), or many (polyclads); those with gonads are hermaphroditic; some have bizarre copulation
- f) recent evidence suggests acoels may be the sister taxon to the Bilateria!
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Copyright (c) 2009 by A. Richard Palmer. All rights reserved.
(revised Dec. 28, 2008)