Mollusca

__**Mollusca

The**__ **phylum of mollusca is made up of soft-bodied invertebrates that usually have a shell, foot, mantle, and a visceral mass. They are found in all environments. Humans are very fond of eating animals from this group. Mollusks have a soft body, usually covered by a hard shell. The soft body is covered and protected by a mantle. The mantle, a thin tissue, secretes the shell for many mollusks. The mollusk has three body parts: head, foot, and a visceral mass. The head contains the brain and sensory organs, the foot is the muscular part of the body and may be divided into parts. The visceral mass area is the space in which the body organs are located.

Most mollusks have well-developed organs that form various systems. They have an open circulatory system. The blood is not always inside the blood vessels. It fills the open spaces in the body and bathes the organs and then flows back to the heart(s). Mollusks have a well defined digestive, respiratory, reproductive, and nervous system. Some mollusks are filter feeders, some use a radula to scrape food from leaves and rocks, and some use tentacles to catch and hold their prey while their beak tears the food apart.

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Mollusks are divided into three major groups.

Gastropods: no shell or one shell (slugs and snails) -radula for scraping food - gliding foot on mucus layer - may be terrestrial

Bivalves: two shells (oysters, mussels, clams) -filter feeders -digging foot

Cephalopods: no shell or internal pen (squid, octopus, nautilus) -predators with beak - tentacles with suckers**