It’s hard to understand the Mollusks, when they’re as different as an oyster and an octopus! Basically, Dad explains, a mollusk is a soft-bodied animal with a shell on the outside (sometimes on the inside!), and strong muscles that hold the shell closed, or clamp the animal firmly against the rocks. Clams, mussels, snails, sea slugs, squid and even the smart octopus, are all mollusks.
“There are bivales all around us,” Dad points out. They live inside double, hinged shells. Once I watched a clam feeding, in a touch tray at the Marine Science Center. [Photo right] Two siphons stuck out between the shells, one taking in water and the other pumping it out. The clam was filtering food in the form of plankton.  When a bivalve is bothered by something, the shells close fast like hands clapping.
We find plenty of mussels; they like wave action because waves and surge bring them food. “Want to see how they tie themselves down to the rocks?” Dad asks. We peer into a big clump of mussels and he points at the thick, short threads stretching between the shells and the rock. No wonder mussels don’t get washed away easily! Dad calls them byssal threads	Mussels siphoning plankton
“Now we’ve seen bivalves; can you guess what a univalve might be?” “I know! Not two shells, just one!” We find lots of marine snails living in a single shell. Some are cone-shaped; these are the limpets. But most shells are shaped like spirals. Dad explains that snails eat by scraping algae off the rocks with a very rough tongue called a radula. It’s so rough that it can bore a hole in another animal’s shell	Marine snails
We find a leafy hornmouth [Photo right], a top shell, many turban snails, and a strange one called a chiton that has 8 hard plates lined up down its back. All are clamped down hard on the rocks, waiting for high tide to return.
Of all the animals we discover, my favorites are the nudibranchs. They hide under seaweeds, which is too bad because most come in beautiful colors. “A nudibranch,” says Dad, “is just a snail with no shell to hide in. Nudibranch means ‘naked gill.’ See?” He points at a bright yellow one that we’ve discovered under some sea lettuce. “The gill, that little plume on its back, is carried outside, unprotected.” I wonder out loud how the nudibranch keeps from being eaten. Dad says it’s very poisonous, and the bright colors are really an advertisement, a warning to its enemies that says “Touch Me Not! I taste awful!”	Lemon nudibranch
“Ho!” Dad shouts suddenly. “I found an octopus!” [Drawing right] He really has! It’s very small, and is probably trapped in the tide pool. We watch it creep slowly across the bottom, its tentacles feeling around under the pebbles, looking for food. “It’s hunting for small mollusks and crustaceans.” I watch the tiny octopus change its color, now dark red, now brown, now white-speckled. “The octopus is a lot like you,” Dad says, laughing. “It’s moody sometimes. Know how you blush red when you’re embarrassed? The octopus has a color and pattern for every mood. These costume changes are also handy for camouflage against different backgrounds.” A smart animal, the octopus! “But how is it related to snails?” I ask. Dad says the octopus and squid are both mollusks, but the squid carries its shell inside its body, and the octopus has lost it altogether, just like the nudibranch has.
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