Class notes
Phylum Mollusca
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Lecture notes from the first year about molluscs (part of Bilaterians, Molluscs and Arthropods)
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Lecture 3: Phylum Mollusca (part of Lophotrochozoans)Includes 100000 species, second most diverse after arthropods. Most molluscs are marine, though
some live in freshwater or land. They're soft bodied animals but most are protected by hard shell of
CaCO3. slugs, squids and octopuses have reduced or lost their shells during evolution.
Molluscs are protostomes.
Archenteron: endoderm (yellow) lined cavity at gasturalation that develops into digestive tracts
Coelom: body cavity lined with mesoderm (red)
Blastopore: opening in archenteron, later a second opening forms to give digestive tube.
What have molluscs evolved from?
Many annelids and molluscs share patterns of development in early embryonic stages. Both have
microscopic swimmers known as a trochophore larva. Larvae of these groups are almost identical,
even though they mature into different forms. Until recently, arthropods were considered close
relatives of annelids, because they both are segmented, but arthropods don’t have trochophore
larvae.
Shared characteristics of lophotrochozoans and ecdysozoans
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