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PHYLUM MOLLUSCA

Second largest phylum behind Arthropoda

- soft bodies enclosed in a calcium carbonate shell

General characteristics:

- complete digestive tract- open circulatory system

- usually separate sexes

- cephalization (having a head)- many are benthic

- mantle (secrets shell)

- unsegmented, bilateral symmetry

- muscular foot – locomotion

- gills

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1. Class: Gastropoda (stomach foot) or Univalvia (one shell)

Their bodies are covered by one shell, except for the nudibranch.

Special tongue (radula) with a ribbon of flexible teeth on its surface works like a mini-chain saw. It is used to scrape surfaces for algae or to cut into other shells.

Most are benthic herbivores, although the dog whelk is carnivorous.

ex: snail, abalone, conch, limpet, nudibranch (no shell)

Spanish Dancer

Figure 7.19

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2. Class: Pelecypoda (hatchet foot) or Bivalvia (two shells)ex: clams, oysters, scallops

- some are sessile (oyster) others swim by opening and closing both shells (scallop)

- benthic planktivores

- filter seawater through incurrent& excurrent siphons

- enclosed in 2 shells(lined by mantle)- body laterally compressed

- no radula

- gills also filter small food particles

Swimming Bivalves

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CLAM DISSECTION Internal Structure of the Clam - Part I Internal Structure of the Clam - Part II Internal Structure of the Clam - Part III

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3. Class: Cephalopoda (head foot) is the basic body plan for squid, octopus, cuttlefish, and chambered nautilus

- the most advanced brain of all invertebrates

- squid has an internal shell (pen) that is clear and flexible

The only hard part of their body is their beak.

- complex nervous system / large eyes / excellent vision

- foot modified into arms and tentacles

- protected by thick muscular mantle

-excellent swimmers – jet propulsion

- hunters

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- mouth has a sharp beak for eating worms, fish, etc. - moves by jet-propulsion, mantle fills with water which can be squirted out to propel them backwards- chromatophores in the skin change colors when the animal is

alarmed and for camouflage- Octopus - Dwarf Cuttlefish - Cuttlefish

- ink gland is used as a “smoke screen” for a quick escape- eyes have a cornea, lens and retina; excellent vision

In the octopus, reproduction is by internal fertilization. During the 30 days until hatching, the female carefully aerates, cleans and watches the eggs - never leaving - even to eat.

The female usually dies from exhaustion and starvation soon after they hatch.

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A soft, flexible body (no shell) allows the octopus to hide in very small spaces.

They are very shy creatures and do not attack humans nor eat ships! In captivity, they seek out human contact.

The octopus’ head has 8 regenerative arms, thus the name octopus. The squid and cuttlefish also have 2 long, tentacles with flattened paddles.

Beak-like jaws, secrete paralyzing substance, some have toxic bite

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The cuttlefish has an internal shell that is sold to pet stores as a source of calcium for birds.Cuttlefish Video

Cuttlefish

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The spiraled shell of the chambered nautilus is lined with mother-of-pearl and is perfectly proportioned mathematically.

Gas and liquid exchange occurs through the walls and allows the nautilus to regulate buoyancy.

If an irritant such as a grain of sand gets trapped under the oyster’s mantle, it secretes a coating of a pearl-like material called nacre. Selected oysters are carefully raised to produce pearls of various sizes and colors.

Oysters are at the center of the aquaculture industry and are raised for culturing pearls.

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Squid Internal Anatomy