Phylum Mollusca Gastropods, Cephalopods, and Bivalves

Phylum Mollusca includes snails, slugs, clams, oysters, squid, cuttlefish, and octopi!
The 4 Big Picture Characterisitcs.

This phylum shows bilateral symmetry due to the presence of a head.

This phylum's embryonic developement shows that they have all 3 layers (ectoderm, mesoderm, and endoderm), so they are triploblastic. They are triploblastic COELOMATES like us humans.

This phylum shows very obvious cephalization (having a head.)

This phylum shows no segmentation.

Main body parts

Foot: used for locomotion. Appears different in each class.

Mantle: Very thin and delicate tissue that covers the body like a cloak.

Shell: Usually made of calcium carbonate (CaCO3). Shell is secreted by the mantle. Shell builds bigger as the mollusk grows.

Visceral mass: Below the mantle. Contains all of the internal organs including the gills.

Radula: A rasping, ribbon-like tongue. Has a sandpaper-like texture.


The radula are rasping, ribbon-like teeth that mollusks use to either scrape algae off of surfaces, like rocks, or use them to drill into the hard exterior of their chosen prey.

For example, most if not all snails will use their radula to scrape algae from rocks.

In the mean time, mollusks like conch will drill holes into the shells of clams and oysters (also mollusks) and then eat their prey through the hole they made.

Some radula can be poisonous and dangerous for humans. Scientific research also believes we can use it to produce medicine.

Watch the radula in action!! Notice the "rasping" tongue and how it is always reaching out for potential food. If it can grab onto some tasty stuff, it will essentially tear it apart with that radula similar to sandpaper.

Respiratory System

Mollusks use gills to breathe (and sometimes to capture food.) Gills are located in the mantle cavity where they can process out O2 from the water.

Circulatory System

Mollusks show both types of circulation: Open and Closed.

Open Circulatory System: Blood mixes with other fluid in the body before waste is removed. It is pumped through the body by a heart. In other words, nutrients and oxygen molecules mix in the body before leftover waste is removed thanks to the excretory system and pumping action of the heart.

Gastropods and Bivalves have the OCS.

Closed Circulatory System: Blood does NOT mix with other fluids. It stays in a loop in the body and is pumped by the heart. This means that nutrients and oxygen do not mix together.

Cephalopods have a CCS (and humans too!)

Excretory System

Mollusks have a complete digestive tract because they have both a mouth and anus. These openings can sometimes be referred to as siphons.

Essentially, ammonia wastes from digestion are similar to urine and can be toxic. They are removed by the nephridia (a more evolved version of nephridiapores we saw in platyhelminthes) which are simple, tube shaped organs leading out of the body via the skin.

Reproductive System

In water environments, egg and sperm are often released in massive quantities. Organisms simply hope that fertilization occurs. Larvae will develop and then swim to a place on the bottom of the watery environment and grow into bivalves.

Where bivalves usually rely on hope alone, snails and cephalopods can perform internal fertilization and even lay eggs.

Mollusks can have spontaneous sex changes when there are not enough members of the necessary sex.

Class Gastropoda

Gastro stands for stomach. Poda stands for foot.

Organisms include Land and Sea Snails, Sluds, and Nudibranchs!

This phylum includes organisms with one shell (and sometimes no shell such as sea slugs.)

Gastropods have a broad, muscular foot on the ventral side of the body.

Shells show RADIAL TORSION, which gives the shell its spiral shape.

Class Bivalvia

Bi means 2. Valve means a hinged, movable part. These animals have 2 shells!

Organisms include oysters, scallops, clams, and mussels.

Bivalves have gills used for breathing, but can also use them to feed! They will extend them out to "screen," or check, the water for microorganisms and algae to munch on.

Bivalves are mostly sessile, but can use their foot to move away from predators and even "swim" when they quickly open and close their shells.

Oysters are bivalves that can create pearls!


Cephalo means head. Poda means foot. This means that these organisms have a combination head and foot! Since their body looks like it is their entire head, they were deemed "head foot" or CEPHALOPODA!

Class Cephalopoda includes Cuttlefish, Squid, Ammonites, Nautilus, and Octopus.

These organisms are HIGHLY mobile. They can move very quickly thanks to their ability to use jet propulsion. Cephalopods can easily pull water in through a siphon and push it back out to propel themselves.

These organisms lost their shells so that they could gain stronger muscles to swim faster. The shell is now internal and, in some species, is found as a "pen."

These organisms are incredibly intelligent with excellent sense, including the eyes.

These organisms can use ink as a defense mechanism.

These organisms have the ability to change their texture, size, and color thanks to chromatophores.

Created By
Jessica Zeran


Created with images by prilfish - "Flamboyant cuttlefish" • BioDivLibrary - "n56_w1150" • Elsie esq. - "An Ammonite" • Joevet - "bone teeth jaw" • robgrowler - "Squid Dissection 3" • Double--M - "Anatomical drawing of abdomen, circa 1900" • fdecomite - "Snails" • fda54 - "staircase snail lighthouse" • stux - "sea snail cineraria trochidae" • fdecomite - "Snails" • fdecomite - "Snails" • joakant - "slug snail diving" • prilfish - "Punk Chromodoris strigata (not 100% sure)" • prilfish - "Juvenile spanish Dancer - Hexabranchus sanguineus" • prilfish - "Risbecia tryoni" • arhnue - "nudibranch chromodoris slug" • James St. John - "Janthina janthina (purple sea snail) 2" • - "Chromodoris Orientalis" • Aaron E. Silvers - "Banana Slug!" • Jim Whitehead - "Banana Slug" • Hitchster - "Banana slug" • Elias Levy - "Nudibranch" • southoz - "nudibranch" • Elias Levy - "Nudibranch" • vjacob1 - "nudibranch thailand sea" • duncan - "nudibranch snorkeling diving" • Zaheer Baksh Photography - "Chip Chip" • PublicDomainPictures - "clam shells molluscs" • S. Rae - "Arctica islandica (Ocean quahog)" • James St. John - "Opalized fossil bivalve (Bulldog Shale, Lower Cretaceous; Coober Pedy Opal Field, South Australia) 08" • DiegoConti - "shell sand sea" • James St. John - "Caribachlamys ornata (ornate scallop) (San Salvador Island, Bahamas) 2" • S. Rae - "Cockle shells" • Arn@ud Ab@die - "Fan mussel (Pinna nobilis)" • Rusty Clark ~ 100K Photos - "Atkins Farms Country Market, Amherst MA" • James St. John - "Opalized fossil bivalve (Bulldog Shale, Lower Cretaceous; Coober Pedy Opal Field, South Australia) 13" • James St. John - "Caribachlamys sentis (scaly scallop) (San Salvador Island, Bahamas) 4" • BioDivLibrary - "n61_w1150" • bob in swamp - "Random Estuarine Beach Scene" • BioDivLibrary - "n484_w1150" • izik - "Clams!" • NeuPaddy - "shell pearl valuable" • NOAA Photo Library - "fish4524" • miriamart - "seashell scallop nature" • andrewmalone - "Mussel pile" • Andy Gant - "Mussel" • LisaW123 - "Mussels" • BioDivLibrary - "n19_w1150" • Cocayhi - "Cuttlefish" • Joi - "Cuttlefish" • prilfish - "Cuttlefish" • andrewmalone - "Cuttlefish" • prilfish - "Flamboyant cuttlefish" • Kačka a Ondra - "Cuttlefish" • NeilsPhotography - "Flamboyant Cuttlefish Macro" • Joi - "Cuttlefish" • jurvetson - "Eye of Octopus" • arhnue - "cuttlefish cephalopod nature" • BioDivLibrary - "n88_w1150" • aitoff - "street art ammonite" • prilfish - "Red Sea Octopus" • WikimediaImages - "asteroceras fossil early" • edenpictures - "Ammonite" • prilfish - "Red Sea Octopus" • ghost of anja - "Nautilus" • James St. John - "Tornoceras uniangulare aldenense fossil goniatite (Alden Pyrite Bed, Ludlowville Formation, Middle Devonian; western New York State, USA) 1" • Elias Levy - "Octopus" • glucosala - "fish octopus water" • vidalia_11 - "Giant Pacific Octopus" • MartinStr - "squid octopus underwater" • ftomiz - "Squid" • prilfish - "Squid" • Hemi - "squid octopus underwater" • Luvina 04 - "Squid" • Anderson Mancini - "Squid" • NOAA Photo Library - "sanc1686"

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