Mollusks

Niño A.CrisoloII-BSE

What Is a Mollusk?Mollusks are soft-bodied animals that usually

have an internal or external shell.

Mollusks include snails, slugs, clams, squids,

and octopi.

Many mollusks share similar developmental stages.

Wide size range (slugs – giant squids)Many shared characteristics among

creatures that appear to be very diverse.Two body parts head-foot & visceral

mass-Two body parts head-foot & visceral mass

-Mantle covers visceral mass, secretes shell

-Bilateral symmetry

-Reduced coelom contains only heart, gonads, & nephridia

-Most have open circulatory system

-Most have radula used for obtaining food

Many aquatic molluskshave a free-swimminglarval stage called atrochophore.The trochophore larva isalso characteristic ofannelids, indicating thatthese two groups maybe closely related.

Form and Function in Mollusks

Mollusks have true coeloms surrounded by mesoderm tissue.

They have complex, interrelated organ systems that function together to maintain the body as a whole.

What is the basic body plan of mollusks?

Body Plan

The body plan of most mollusks has four

parts: foot, mantle, shell, and visceral

mass.

The muscular foot takes many forms

flat structures for crawling spade-shaped structures for

burrowing

tentacles for capturing prey

SnailClam

Squid

The mantle is a thin layer of tissue that covers most of the mollusk's body.

The shell is made by glands in the mantle that

secrete calcium carbonate.

Just beneath the mantle is the visceral mass, which contains the internal organs.

Feeding

Mollusks can be herbivores, carnivores, filter

feeders, detritivores, or parasites.

Snails and slugs feedusing a flexible, tongueshapedstructure knownas a radula.

Hundreds of tiny teeth are attached to the radula.

The radula is used toscrape algae off rocks or to eat the soft tissues of plants.

Clams, oysters, and scallops use gills.

Food is carried by water, which enters the incurrent siphon.

A siphon is a tube like structure through which water enters and leaves the body.

The water flows over the gills and leaves by the excurrent siphon.

Respiration

Aquatic mollusks breathe using gills inside their mantle cavity.As water passes through the mantle cavity,oxygen in the water moves into blood flowing through the gills.At the same time, carbon dioxide moves in the opposite direction.

CirculationSome mollusks have open circulatory systems;

other mollusks have closed circulatory systems. In an open circulatory system, blood is pumped

through vessels by a simple heart.Blood leaves the vessels and works its way through

different sinuses.Blood passes from the sinuses to the gills, where

oxygen and carbon dioxide are exchanged. Blood is then pumped back to the heart.

Slow-moving mollusks often have open circulatory systems.

Faster-moving mollusks have a closed circulatory system.

A closed circulatory system can transport blood through an animal’s body much more quickly than an open circulatory system.

Groups of Mollusks

Gastropods (SNAILS, WELKS)

Bivalves (CLAMS, OYSTERS, MUSSELS)

Cephalopods (OCTOPUS & SQUID)

The three major classes of mollusks are

Gastropods

Gastropods are shell-less or single shelled mollusks that move by using a muscular foot

located on the ventral side.Many gastropods have a single shell that

protects their bodies.When threatened, they can pull completely into their shells.

Name comes from Greek– Gaster = “gut”, podos = “foot”

Gastropods include

pond snailsland slugssea butterfliessea haresLimpetsnudibranchs

GASTROPODA

Bivalves

Common bivalves include:

• clams

• oysters

• mussels

• scallops

Bivalves have two shells that are held together by one or two powerful muscles.

Class Bivalvia (CARD)Name comes from Latin– Bis = “twice”; valva = “leaf”

• 2nd largest class of mollusks• Includes clams, oysters, scallops, mussels• Many are edible• Virtually all marine or freshwater– Eat via filter feeding

Bivalve Shell

• The two halves of the shell are called valves– Shell forms in one piece during development = singlestructure, not a pair of shells– Shell secreted by mantle mantle secretes extraprotein at the dorsal area that will become the hingeligament• Ventral side has “teeth” that fit together and keepthe halves lined up properly• Umbo = oldest part of shell at anterior end;swollen appearance

Cephalopods

• Name comes from Latin & Greek– Cephalic = “head”; podos = “foot”

• Most complex class of mollusks• Includes octopuses, squid, cuttlefish, chambered nautilis• Anterior portion of foot modified into tentacles– Tentacles arranged in a circular pattern– Used for capturing prey, attachment, movement, copulation

Cephalopods Cephalopods are typically soft-bodied mollusks in which

the head is attached to a single foot. The foot is divided into tentacles or arms.

Cephalopods have eight or more tentacles equipped with sucking disks that grab and hold prey.

Most modern cephalopods have only small internal

shells or no shells at all. The only present-day cephalopods with external

shells are nautiluses.

Cephalopods have complex sense organs that help them detect and respond to external stimuli.

Cephalopods distinguish shapes by sight and texture by touch.

The eyes of many cephalopods are complex.

CEPHALOPODS

• SQUID• OCTOPUS

General Facts

50,000 – 200,000 species (many remainundiscovered)• Evolved ~500 million years ago.• Oldest resemble today’s Nautilis.• Fossilize well – hard shell• Mollusk shell grows entire life – rings like a tree trunk• Many land snails can lift 10x’s own weight.• 1 acre of cultivated mussels can produce~10,000 pounds of meat/year – 500x’s more that an acreof pasture can produce beef.