Mollusks

1. Mollusks Phylum Mollusca

2. What is a mollusk? • The word “mollusk” comes from a Latin word meaning “soft.” • A mollusk is a soft-bodied animal that usually has an internal or external shell.

3. Characteristics • There are four main parts to a mollusk: foot, mantle, shell, and visceral mass. • There are a variety of feeding patterns. • Various mollusks eat just about anything, and they often have a radula, a flexible tongue-like structure covered by teeth. • They range from aggressive in attacking prey (like the octopus) to filter feeding (like clams, oysters, and scallops. • They may have gills or use their mantle cavity for breathing. • They may have open or closed circulation. • In an open circulatory system, blood leaves vessels and moves through body spaces. • In a closed circulatory system, it stays in vessels for efficiency. • Tube-shaped nephridia excrete ammonia from the body.

4. Nervous systems vary widely. • Clams and their relatives have a simple nervous system consisting of small ganglia, a few nerve cords, and simple sense organs. • Octopi and their relatives have a well-developed brain and are the most intelligent invertebrates. • They range from slow-moving (snail) to fast-moving (octopus). • Sexual reproduction may be external or internal, depending on the type of mollusk.

5. Types of Mollusk • Class Gastropoda • These are shell-less or single-shelled mollusks that move using a muscular ventral foot. • examples: snail, slug • Those that have shells can pull into them for protection. • Those that lack shells hide under something. • Others have toxins that can be used if threatened. • Class Bivalvia • These have two shells held together by one or two powerful muscles. (examples: clam, oyster, mussel, scallop) • They are filter feeders and use mucus and cilia to trap food particles.

6. Class Cephalopoda • These are soft-bodied mollusks with the head attached to the foot, which is divided into tentacles or arms. • examples: octopus, squid, nautilus • They have little or no shell remaining, except for the nautilus. • The shell is modified into a rod in the squid and is completely gone in the octopus. • They have complex sense organs, including large eyes.

7. Ecological Issues • Mollusks are important in recycling in the biosphere as well as a food source for humans. • Some mollusks have been found living in the deep-sea vent community with chemosynthetic bacteria. • Mollusks can be used for checking water quality and in other biological research.