Mollusks
After insects, mollusks are the largest invertebrate group. They include gastripods, such as slugs and snails;
bivalves, such as clams and oysters; and cephalods, such as cuttlefish, octopuses, and squid. Many have a hard shell, and feed with a rasping, toothed tongue.
The snail has a single lung near the opening of the shell. It glides over a sticky mucous produced by a salivary glands. When the snail eats its food, its food is stored in its crop and then digested in its stomach. Its reproductive system often has both male and female organs.
Octopuses and squid are cpehalopods (mollusks with a ring of arms). Their arms have suckers, which they use to catch prey. They have large eyes, and the best-developed nervous system of all invertebrates. Most swim by jet propulsion, and can shoot backward at high speed.
Octipuses have eight arms and hard, beaklike jaws. They move by crawling on the seabed. They can change color for camouflage or communication. A squid has an internal shell, eight arms, and two tentacles.
Clams and oysters are bivalve mollusks. Their shell is divided into two halves (or valves), which hinge together. Bivalves are found in marine and freshwater habitats, but not on land. They breathe using large gills, which they also use for feeding by filtering food particles from the sea. Clams usually live buried in mud or sand, and move by burrowing. Oysters live anchored to rocks and cannot move about,
Oysters fasten one side of their shell to a rock, or to another oyster. They open their shell to feedm but close it if touched. Giant clams live on coral reefs and filter food from the water. Many clam species feed by sucking water into their gills through a long tube.
The largest mollusk is the giant squidm which can be 65 feet (20m) long. About hald of this length is its tentacles.
The giant clam is the largest bivalve mollusk, Despite many stories claiming that it traps people by their feet, this has never been known to happen.
The most dangerous mollusks are tropical cone shells, which kill with a venomous “harpoon.” Their sting can kill humans.