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Name ______________________________________________________________ Introduction to Mollusks There are over 50,000 known species of Mollusks, which makes them second only to the Arthropods in invertebrate phylum size. There are three classes of Mollusks – Cephalopods (head-­foot), Gastropods (stomach-­foot), and Bivalves (2-­shells). Class Examples Cephalopoda Gastropoda Bivalvia Mollusks have bilateral symmetry, meaning that the left and right sides of their body are symmetrical. A soft body and presence of a hard shell are the defining features of the phylum of Mollusks. Additionally, all mollusks have three structures in common: mantle, shell, and foot. The mantle is the organ that creates the shell and attaches the mollusk to the shell. The mantle cavity, or space inside the mantle, contains all of the internal organs. The shell provides protection and support for the mollusk and grows with the organism throughout its life. The foot is a large muscle that helps the mollusk move in its environment. In gastropods, it is found on the bottom of the organism, in bivalves it is contained inside the shell, and in cephalopods the foot has been modified into tentacles that can grasp and manipulate food. Cephalopods and Gastropods also have a structure called a radula. This structure is found in the mouth and helps the mollusk break its food down into smaller pieces – think of it as a tongue covered in sand paper. By licking their food with their rough tongue, they break it down into small pieces. Remember the grazing periwinkles on the rocks? They use their radula to scrape the algae off the rocks. Bivalves, on the other hand, are filter feeders. They pull water into their bodies through the siphon and use their gills to strain small food particles out of the water. Shape of Life Video 1. What are the main structures of mollusks? 2. How does the foot help the Abalone move? 3. What does the Cockle’s foot help it do? 4. What is the job of the radula? 5. What were the EXTERNAL evolutionary forces for the shell and the radula? 6. How did the predators of ancient mollusks push mollusk evolution beyond the shell and radula? 7. What structure did squids lose when they evolved to be faster swimmers? 8. What are some of the “advanced” internal structures in the squid? 9. Why was it risky for octopuses to return to living on the bottom of the ocean? 10. What is the octopus’ best defense?