Molluscs (Ph. Mollusca)
- Present variations on a common body plan, with a muscular foot, a mantle that secretes the shell, a visceral mass, and a radula, a feeding organ unique to the phylum.
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They are bilateral protostome coelomates, and comprise the second-most diverse animal phylum (150,000 sp.).
- Three major classes are:
- Gastropoda (snails, slugs), crawling grazers with a broad foot and spiral shell
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Bivalvia (clams, mussels), sedentary filter feeders with large gills and two shells -
Cephalopods (squid, octopuses). Active, swimming predators with a foot modified to tentacles, and a reduced/lost shell
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Molluscs show the greatest variation in body form, with both closed- and open circulatory systems (depending on activity level), highly specialized organs, and loss of characters among groups (shell, radula). -
Important economically as food species, ecologically as components of marine food chains, and scientifically for studies of learning and behavior.
True segmentation involves repetition of body regions
- Must also include associated organ systems.
segmentation is an easy way to build a larger animal and allows greater flexibility and mobility; the form probably evolved to facilitate movement -
Some organ systems (digestive, reproductive) differ among segments. -
In the evolution of some groups, segments become joined into larger body regions, with internal structures also combined.
Annelids (Ph. Annelida)
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Characteristics: represent the truly segmented worms, and most possess short hairs (chaetae). -
They are bilateral protostome coelomates, with probably the most obvious coelom of any phylum. -
They are moderately diverse, with about 15,000 sp. -
Three major groups are the polychaetes (Polychaeta: clam- and tube worms, giant hydrothermal vent worms), the earthworms (Oligochaeta) and the leeches (Hirudinea). -
A few leeches are parasitic and important in medicine, but most annelids are important decomposers or bottom feeders.
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