A. Phylum Mollusca Is the Second Largest Animal Phyla After Arthropoda
a. Phylum Mollusca is the second largest animal phyla after Arthropoda. There are over 90,000 described living species and some 70,000 fossil species. b. Their name comes from the Latin word ‘molluscus’ meaning soft, after their soft bodies. Although in many species, their soft body is protected by a shell. c. Mollusks are a diverse group that range from fairly simple organisms to some of the most complex and specialized of the invertebrates. They include chitons, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, scallops, cuttlefish, squid, octopuses, and nautiluses. d. Mollusks are found in a great range of habitats, although most require water. Only snails and slugs are terrestrial, yet they too are limited in their range by their need for humidity, shelter, and presence of calcium. e. Mollusks are triploblastic coelomate protostome invertebrates. i. While the molluscan coelom is relatively small, the development of the coelom was a major step in the evolution of larger and more complex forms of animals. a. There are seven living classes of Mollusks: i. Polyplacophora 1. Contains 600 species of chitons found on rocky marine shorelines. ii. Gastropoda 1. Contains an estimated 40,000 - 150,000 species of nudibranchs, snails and slugs. Also includes unique species such as limpets, sea hares, sea angel, sea butterfly, and Sea Lemon (pictured above). iii.Bivalvia 1. Also called Pelecypoda; contains 8,000 species of clams, oysters, scallops, and mussels. iv.Cephalopoda 1. Contains 786 species of squid, octopus, nautilus, and cuttlefish. v. Monoplacophora, Aplacophora and Scaphapoda are rare and/or extremely deep-water creatures, and will not be discussed.
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