MOLLUSKS

Soft-bodied invertebrates, mollusks include slugs, snails, octopuses, squid, clams, and mussels. Most mollusks have SHELLS to protect them.

WHAT FEATURES DO MOLLUSKS HAVE IN COMMON?

As well as a shell, most mollusks have a muscular foot for creeping or burrowing. Some also have a head with sense organs. The soft body includes lungs or gills for breathing, and digestive and reproductive parts, all enclosed by a skinlike organ called the mantle.

HOW DO MOLLUSKS FEED?

Most mollusks have a rasping tongue called a radula, armed with tiny teeth. This scrapes tiny plants and animals off rocks or tears food into chunks. Bivalves, such as oysters and mussels, filter food particles from the water with their gills.

OCTOPUS IN ACTION

Cephalopods such as octopuses and cuttlefish are stealthy hunters. Octopuses creep along the seabed or lie in wait for fish and crabs. They pounce on their prey, seize it with their suckered arms, and paralyze it with poisonous saliva.

HOW DO MOLLUSKS REPRODUCE?

Mollusks reproduce sexually. Slugs and snails are hermaphrodites (possessing both male and female organs), but they must still mate to fertilize their eggs. Most aquatic mollusks lay eggs that hatch into small, free-swimming larvae called veliger.

MOLLUSK CLASSIFICATION

There are over 70,000 mollusk species split into several major classes:

Gastropods (the largest class) include slugs, snails, winkles, whelks, and limpets
Bivalves include scallops, clams, and oysters
Squid, octopuses, cuttlefish, and nautilus are cephalopods
Smaller groups include tusk shells and chitons (oval mollusks with jointed plates)

SHELLS

Mollusk shells come in many shapes and sizes, but most have the same, simple function—providing somewhere to hide in times of danger. In land mollusks, the shell also helps to prevent the moist, soft-bodied creature from drying out.

WHAT ARE MOLLUSK SHELLS MADE OF?

Mollusk shells are made of a chalky material called calcium carbonate. The shell has three layers for extra strength: a tough outer layer, a chalky middle layer, and a shiny inner layer, next to the animal’s skin. The shiny layer in some bivalve mollusks is known as mother-of-pearl.

HOW DO MOLLUSKS MAKE SHELLS?

A mollusk’s mantle (skin) releases liquid shell materials, which harden on contact with water or air. Gastropod and nautilus shells grow from their outermost edge. As the mollusk grows, its shell develops more whorls (single turns in a spiral shell) or chambers. In bivalves, new shell material is deposited on the edge that is farthest from the hinge.

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Invertebrates

Copyright © 2007 Dorling Kindersley