**Mollusks** are members of the phylum Mollusca. These species are characterized by having a soft body, which usually has an internal or external shell made up of calcium carbonate, secreted by their body.

The foot of a mollusk is used for locomotion or movement. It can appear as tentacles, flat or spade-shaped structures.

The mantle of a mollusk covers the body of the mollusk.

The shell of a mollusk is composed of the glands in the mantle that produces calcium carbonate.

The visceral mass is located beneath the mantle. It holds the organs in the digestive, productive, excretory, and respiratory systems.

**Mollusks** are members of the phylum Mollusca. These species are characterized by having a soft body, which usually has an internal or external shell made up of calcium carbonate, secreted by their body. There are three major groups of mollusks: *Gastropods, Bivalves*, and *Cephalopods*.

**Gastropods** are members of the class Gastropods, that are characterized by the presence of soft shells, and locomotion using a muscular foot located in the ventral portion. Example species are snails, slugs, and limpets.

**Bivalves** are members of the class Bivalvia. These species are composed of two hard shells that are closely held together by one or more powerful muscular structures. They have cilia on their gills which are used for their feeding mechanism. Example species: clams, oysters, and mussels.

**Cephalopods** are members of class Cephalophoda. These organisms are usually soft-bodied having a head attached to their sole foot. The foot then contains many tentacles acting as their arms. Their tentacles are usually equipped with sucking discs that function to grab and hold their prey. Example species: Squids and octopus.

Snails exchange oxygen with carbon dioxide inside the blood vessels located mantle cavity. In order to do so, the mantle cavity lining must be kept moist for the movement of gases to occur. This is why snails cannot survive on dry land.

Cephalopods have complex sense organs that are similar to some fishes and other vertebrates. These sense organs are able to detect and respond to external stimuli through seeing and touching.

The cephalopod’s nervous system is considered to be more developed compared to other invertebrates. They have larger brains when compared to the brains of the gastropods. They have advanced visual senses that are able to detect preys and predators. Because of these conditions, they are regarded as the most intelligent of the invertebrates.

In an **open circulatory system**, the circulatory fluid leaves the heart and enters body cavities, bathing the organs. In a **closed circulatory system**, the circulatory fluid is closed in vessels; the fluid cannot exit unless the vessel is broken. A closed circulatory system is less efficient which fits for slow-moving organisms.

See explanation.