Mollusk Glossary/Terminology

A mollusk may be an octopus, squid, or cuttlefish. It also may be an oyster, clam, nudibranch, snail or one of thousands of other very different species.  Learning about mollusks? Here are some terms you might encounter, along with their definitions.

Scallop / Ryoji Yoshimoto/Aflo / Getty Images
Scallop, showing adductor muscle. Ryoji Yoshimoto/Aflo / Getty Images

Did you know that when you eat a scallop, you're eating the adductor muscle? In mollusks, the adductor muscle (or muscles, in some cases) is the muscles that works in conjunction with a ligament to open and close the bivalves two valves, or shells.  More »

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Apex

Lightning Whelks / Bob Richmond, Flickr
Lightning Whelks, Busycon sp. Courtesy Bob Richmond, Flickr

The apex is the very tip on the top (pointy end) of a gastropod's shell. 

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Beak

Squid Beak / Franco Banfi / Getty Images
Sharp Beak of Jumbo Squid, Humboldt Squid, Dosidicus gigas, Santa Rosalia, Sea of Cortez, Baja California, East Pacific, Mexico : Stock PhotoView similar imagesMore from this photographerDownload compSharp Beak of Jumbo Squid, Humboldt Squid, Dosidicus gigas, Santa Rosalia, Sea of Cortez, Baja California, East Pacific, Mexico. Franco Banfi / Getty Images

You're probably familiar with the beak of a bird, but how about the beak of cephalopods? The beak in a cephalopod is made up of two "jaws" within the animal's body. This structure is used for A

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Mussels (Mytilus edulis) feeding, Ireland / Paul Kay / Oxford Scientific / Getty Images
Mussels (Mytilus edulis) feeding, Ireland. Paul Kay / Oxford Scientific / Getty Images

A bivalve is a type of mollusk with two hinged valves, or shells. Examples of bivalves include clams, mussels, oysters and scallops. More »

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Cephalopod

Squid, Sepioteuthis lessoniana, Red Sea, Sinai, Egypt / Reinhard Dirscherl / WaterFrame / Getty Images
Squid, (Sepioteuthis lessoniana), Red Sea, Sinai, Egypt. Reinhard Dirscherl / WaterFrame / Getty Images

A cephalopod is a mollusk that is a member of the class Cephalopoda. This includes the octopuses, cuttlefish, squid and nautilus. 

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Cerata

Spanish Shawl Nudibranch - Flabellina iodinea Image / Jerry Kirkhart, Flickr
Spanish Shawl Nudibranch (Flabellina iodinea). Jerry Kirkhart, Flickr

Cerata are a series of projections found on some nudibranchs. These cerata can be many shapes, including horn, club, or thread-like in appearance. 

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Chiton

Lined Chiton, British Columbia / Jeff Rotman / Photolibrary / Getty Images
Lined Chiton, British Columbia. Jeff Rotman / Photolibrary / Getty Images

If any mollusk can resemble an armadillo, it's a chiton. These oval-shaped mollusks have a shell made up of overlapping plates. You might find these animals holding on tight to rocks in a tide pool. 

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Foot

Venomous Geography Cone Snail / Borut Furlan / WaterFrame / Getty Images
Venomous Geography Cone Snail (Conus geographus), Bunaken Nationalpark, Sulawesi, Indonesia. Borut Furlan / WaterFrame / Getty Images

A foot in a mollusk is a muscular appendage (like that shown in the snail here) that is used for movement or burrowing.  

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Gastropod

Flat Periwinkle (Littorina obtusata) / Fotosearch / Getty Images
Flat Periwinkle (Littorina obtusata), showing tentacles and on top of green seaweed, Eyemouth, Scotland, UK. Fotosearch / Getty Images

In the mollusk world, the term gastropod refers to the class of animals that includes the snails, sea slugs and sea hares.

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Limpets in Tide pool, Baja Mexico / Danita Delimont / Gallo Images / Getty Images
Limpets in Tide pool, Baja Mexico. Danita Delimont / Gallo Images / Getty Images

Many mollusks, such as the limpets shown here, are herbivores - animals that get their nourishment from plants. More »

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Invertebrate

Sea hare feeding on kelp, Cornwall, England / Mark Webster / Lonely Planet Images / Getty Images
Sea hare feeding on kelp, Cornwall, England. Mark Webster / Lonely Planet Images / Getty Images

 All mollusks are invertebrates - they don't have a notochord.

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Malacology

Scuba Diver with Humboldt Squid (Dosidicus gigas) at Night/Franco Banfi / WaterFrame / Getty Images
Scuba Diver with Humboldt Squid (Dosidicus gigas) at Night, Loreto, Sea of Cortez, Baja California, East Pacific, Mexico. Franco Banfi / WaterFrame / Getty Images

Malacology is the study of mollusks. 

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Giant Clam Mantle / Ernest Manewal / Lonely Planet Images / Getty Images
Mantle of a Giant Clam. Ernest Manewal / Lonely Planet Images / Getty Images

The mantle is the part of a mollusk that forms the outer wall of the mollusk's body and encloses the mollusk's visceral mass (internal organs). It also secretes the materials that make the shell in mollusks that have one. The image here shows the colorful mantle of a giant clam. More »

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Operculum

Shell with Operculum / Matthew Ward / Dorling Kindersley / Getty Images
Overhead and underside view of Thorn Latirus shell and Operculum. Matthew Ward / Dorling Kindersley / Getty Images

An operculum is like a trap door that gastropod mollusk uses to seal up its shell. This keeps moisture in and protects the animal inside.

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Oyster with Pearl / Images Bazaar / Getty Images
Oyster with Pearl. Images Bazaar / Getty Images

All shelled mollusks can produce pearls, which are formed from an irritant such as a grain of sand or even a bit of food stuck in between the animal's mantle and shell. To protect itself, the animal coats the particle with shell layers. The particle may become embedded in the shell, or it may not, in which case it becomes a pearl. More »

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Flat Periwinkle (Littorina obtusata) / Fotosearch / Getty Images
Flat Periwinkle (Littorina obtusata), showing tentacles and on top of green seaweed, Eyemouth, Scotland, UK. Fotosearch / Getty Images

A radula is a feeding structure present in all mollusks except bivalves. This structure is covered with teeth and is used for drilling, grating or scraping. More »

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Rhinophore

Spanish Dancer Nudibranch / prilfish, Flickr
Spanish Dancer Nudibranch, showing two rhinophores on the head (to the right). prilfish, Flickr

Rhinophores are hornlike tentacles on the head of a nudibranch that have scent and taste receptors to help the nudibranch detect food or potential mates.These structures can be a variety of shapes, such as horn, club or feather-like in structure. 

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Blue Mussel / Wolfgang Poelzer / WaterFrame / Getty Images
Blue Mussel (Mytilus edulis), Bulgaria. Wolfgang Poelzer / WaterFrame / Getty Images

The term sessile describes animals that does not generally move about freely. An example of a sessile mollusk is the blue mussel. More »

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Spire

Shell with Operculum / Matthew Ward / Dorling Kindersley / Getty Images
Overhead and underside view of Thorn Latirus shell and Operculum. Matthew Ward / Dorling Kindersley / Getty Images

The spire is the part of the gastropod shell that includes all whorls (all full turns of the gastropod's shell) other than the last (body) whorl, which is the whorl which houses the shell's opening. In the snail shells shown here, the spire is the portion of the shell towards the right of the image.

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Mussel Veliger / NOAA OER Gulf of Mexico 2002
A larval mussel illustrating the planktonic larval stage, or veliger. This image shows the delicate tissue above the larva, the velum, which has cilia (moving hairlike structures) that the larva uses for movement and feeding. Courtesy Gulf of Mexico 2002, NOAA/Office of Ocean Exploration and Research

Many mollusks go through a planktonic, larval stage in which they are called a veliger. Is is during this stage that the mollusk's shell (if applicable) and foot develop. 

  More »

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Visceral Mass

The visceral mass refers to the part of a mollusk that contains the organs necessary for digestion, circulation and reproduction. 

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Whorl

Queen Conch Shell / Tami Heilemann, USFWS
Queen Conch Shell. Courtesy Tami Heilemann, U.S. Fish & Wildlife Service

A whorl is a part of a gastropod's shell. It is a complete 360 degree turn of the shell. The last whorl is called the last or body whorl, and this whorl is the one that houses the aperture - the large opening in a gastropod's shell. 

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Kennedy, Jennifer. "Mollusk Glossary/Terminology." ThoughtCo, Aug. 29, 2016, thoughtco.com/mollusk-glossary-terminology-2291913. Kennedy, Jennifer. (2016, August 29). Mollusk Glossary/Terminology. Retrieved from https://www.thoughtco.com/mollusk-glossary-terminology-2291913 Kennedy, Jennifer. "Mollusk Glossary/Terminology." ThoughtCo. https://www.thoughtco.com/mollusk-glossary-terminology-2291913 (accessed November 20, 2017).