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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 >15,000 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) considered the most primitive taxon within the Protostomia:
- have some protostome characteristics (spiral cleavage, blastopore becomes mouth, multiciliated cells)
- lack others (coelom, circulatory system, anus, trochophore larvae)
- c) have a blind gut (no anus); the mouth is primitively mid-ventral but becomes anterior in derived taxa
- d) no circulatory system or coelom (filled with spongy parenchyma)
- e) depend on a network of protonephridia for excretion
- f) have a simple ladder-like nervous system with anterior brain
- g) tremendous capacity for regeneration and asexual reproduction
- h) 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
- i) classification is a problem: the Turbellaria is paraphyletic
- 2) Class TURBELLARIA- mostly free-living flatworms (3,000 spp.)
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Copyright (c) 2004 by A. Richard Palmer. All rights reserved.
(revised Jan. 30, 2004)