Mollusks are soft-bodied animals that usually have an internal or external shell.
- body structure, including the terms foot, mantle, and visceral mass -
The body plan of most mollusks has four parts: foot, mantle, shell, and visceral mass. The muscular foot takes many forms, including flat structures for crawling, spade-shaped structures for burrowing, and tentacles for capturing prey. The mantle is a thin layer of tissue that covers most of the mollusk's body, much like a cloak. The shell is made by glands in the mantle that secrete calcium carbonate. The shell has been reduced or lost in slugs are some other mollusk groups. Just beneath the mantle is the visceral mass, which consists of the internal organs.
- open circulatory system--
In an open circulatory system, blood is pumped through vessels by a simple heart.
- nervous systems/brains of shelled mollusks and non-shelled mollusks-
Clams and other two-shelled mollusks typically lead inactive lives, burrowing in the mud or sand. They have a simple nervous system consisting of small ganglia near the mouth, a few nerve cords, and simple sense organs, such as chemical receptors and eyespots.
In contract, octopi and their relatives, such as the octopus are active and intelligent predators that have the most highly developed nervous system of all invertebrates. Because of their well-developed brains, these animals can remember things for long periods and may be more intelligent than some vertebrates. Octopi are capable of complex behavior, such as opening a jar to get food inside, and they have been trained to perform different tasks for a reward or to avoid punishment.
The three major classes of mollusks are gastropods, bivalves, and cephalopods.
Gastropods include pond snails, land slugs, sea butterflies, sea hares, limpets, and nudibranchs. Gastropods are shell-less or single-shelled mollusks that move by using a muscular foot located on the ventral side.
Common bivalves include clams, oysters, mussels, and scallops. Bivalves have two shells that are held together by one or two powerful muscles.
Cephalopods include octopi, squids, cuttlefishes, and nautiluses. Cephalopods are typically soft-bodied mollusks in which the head is attached to a single foot. The foot is divided into tentacles or arms.