Phylum Molluscs - PowerPoint Presentation

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Phylum Molluscs

Prepared by : Nada H. LubbadSlide2

Introduction

: Soft bodied animals.

- name from Latin word “ mollus” meaning soft. The second largest phylum.

The scientific study of Mollusks is known as Malacology.Slide3

Structure:

Most mollusks are protected by an external, hard shell made of calcium carbonate.

except: - in the octopus & sea slugs is completely absent. - in squids, it’s located internally. Body type : Bilateral symmetry.

Body organization : Triploblastic. Body cavity : reduced coelom contains only heart & gonads.

Body plan

three distinct body zones.

Head-

Foot : contains sensory and motor organs. Visceral mass : contains internal organs used for digestion, excretion, and reproduction. Mantle : a fold of tissue that drapes over the visceral mass and secretes a shell . Slide4

Mantle cavity

: water-filled chamber that houses the gills, anus and excretory pores. * space between the mantle and the visceral mass (body organs).

Functions of mantle: - Protection of underlying loose structures.

- Gas exchange through it’s external surface. - in aquatic types, it has receptors for testing the surrounding waters.Slide5

Shell structure & formation

A pearl formationSlide6

Digestive system:

Complete, one-way digestive tract with a mouth &

anus.Specialized feeding structures : Gills, Beak, Radula .

The radula :It is a rasping, tongue like feeding structure found in most mollusks except bivalves.

Has tiny rows of teeth for scraping.The radula is used to scrape algae off rocks or to eat the soft tissues of plants.

Food is taken up by cells in the lining of the stomach & passed into the blood.

undigested materials are compressed and packaged and discharge through the anus to the mantle cavity and carried into water currents Slide7

Gastropod Feeding ApparatusSlide8
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Excretion system

Excretory functions are carried out by a pair of nephridia.

nephridia – tubular structures that collect fluids from coelom and exchange salts with body tissues as fluids pass along the tubules.

Nervous system

Relatively complex.Most complex - octopus

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2 pairs of main nerve cords.

Visceral – serve the internal organs.

Pedal – control the foot.

Brain encircles the esophagus.

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Generalized Mollusc AnatomySlide11

Respiration and circulation

Open circulatory system ( Gastropoda, Bivalvia, Polyplacophora ).

The blood does not circulate entirely within vessels but is collected by the gills, pumped into the heart, and released in spaces in the tissue ( hemocoel- blood cavity

) and returns to the gills, then the heart.Slow moving lifestyle has lower oxygen demands. Closed ( Cephalopoda ). Heart pumps blood within blood vessels.Active lifestyle, large size has higher oxygen demand and requires more efficient circulation.Slide12

Respiration pigment –

hemocyanin.Respiration is through gills, ctenidia, located in the mantle cavity.Slide13

Reproduction:

All Mollusks reproduce sexually.Depending on the species of mollusk, some reproduce externally or

internally.Dioecious- Have male and female.Some are hermaphroditic. Molluscan Larval

Stages.Most Molluscs produce a free-swimming ciliated larvae called the

trochophore

larvae.

In some

Molluscs

the trochophore develops into the adult, but in other

Molluscs (e.g., gastropods) there is a second larval stage called the veliger.

Trochophore larvaeSlide14

Mollusk Larvae

Trocophore Larvae.

Free-swimming larval stage (first form) Veliger

larvaeFree-swimming larval stage with foot, eyes, tentacles, and shell.

Veliger larvae

Trocophore LarvaeSlide15

Major classes:

Phylum Mollusca

Class

Polyplacophora

Class

Gastropoda

Class

Bivalvia

Class

CephalopodaSlide16

Class Gastropoda

Name comes from Greek – Gaster

= “gut” , podos = “foot

”.Gastropods make up 80 % of Molluscs.Most gastropods have an external shell that protects their bodies.When threatened, they can pull completely into their shells.They can either be herbivores, scavengers, and carnivores.

They usually live in oceans, on rocks and land, and in freshwater.They use their muscular foot to creep along a carpet of mucus that they ooze out in order to crawl.Slide17

Examples:

Sea butterfly

Sea hare

Snail

slug

Sea slugSlide18

Class Bivalvia

Name comes from Latin- Bis =“ twice” , valve= “ leaf “.

Second largest class of mollusks.Bivalves have two shells that are held together by one or two powerful muscles.

Many are edible.Marine or freshwater.Eat via filter feeding- no head or radula.

Incurrent and excurrent siphons are used to pump water through the organism for:

Gas exchange

Filter feeding

Jet propulsionSlide19

Bivalves move around by extending the muscular foot between the shells.

Scallops and file shells swim by clapping their shells together to create jet propulsion.Like other mollusks, bivalves have a coelom and an open circulatory system.

They breathe through gills and filter feed

Examples:

Mussels

scallops

clamsSlide20

Class Cephalopoda

Name comes from Latin & Greek – Cephalic= “head” , podos = “ foot “.

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.They all live in ocean.They are able to move through jet propulsion ( squeezing a current of water out of their mantle cavity ) .Their closed circulatory system makes them the fastest, smartest, and the biggest of all mollusks.Slide21

Most cephalopods have complex eyes with cornea, lens, chambers, and retina.

Largest invertebrate brain.

Protection

Color changes effected by chromatophores (pigment cells).Allows them to blend into their background.

Squirting out water by jet propulsion helps escape predators.

Squids also release an inky substance into the

water.Slide22

Examples:

octopus

squid

nautilusSlide23

Anatomy of squidSlide24