Kingodom AnimaliaPhylum Mollusca: Molluscs

By Natalia Ruggiero and Jacque O'Connell

Structure

● soft bodied animals○ name from Latin word "mollus" meaning soft

● most are protected by a hard shell made of Calcium Carbonate

● have a reduced internal cell or have lost the shell completely during evolution.

● possession of a coelom ○ coelom- a fluid-filled cavity that develops within the

mesoderm

Structure Cont.

● 3 distinct body zones:○ 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 if there is one present ■ mantle cavity- water-filled chamber that houses

the gills, anus, and excretory pores. ● radula- straplike rasping organ used to

scrape up food with curved teeth ○ it allows the molluscs to scrape algae and other food

off of rocks and drill through the shell of prey or catch fish

Digestion and Excretion

● The digestive tract is complete and ciliated with a mouth, anus, and complete stomach.

● Food is taken up by cells in the lining of the stomach and passed into the blood ○ undigested materials are compressed and packaged

and discharged through the anus to the mantle cavity and carried into the water currents.

● 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 tublues.

Nervous System

● relatively complex ○ most complex- Octopus

● 2 pairs of main nerve cords○ visceral- serve the internal organs○ pedal-control the foot

● brain encircles the esophagus ● sensory ability

○ variety of organ systems ○ large, complex eyes

■ larges eyes- large squid(eyes the size of dinner plates)

Respiration and Circulation● open circulatory system

○ 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)and returns to the gills, then the heart. ■ hemocoel- "blood cavity"

● Respiration pigment- hemocyanin● Respiration is through gills, ctenidia, located

in the mantle cavity

Reproduction● sexual ● can by simple or very complex

○ varies in different classes● fertilized egg develops into a swimming form

called a trochophore larva ● this then elongates to become a veliger larva

Locomotion

● varies, but is dictated by the structure of the foot

● Herbivorous forms are gliders, moving on waves of muscular contraction.

● Cephalopods swim actively by jet propulsion in which water is rapidly expelled from the mantle cavity.

● Cuttlefish and Sea Hares rely on lateral fins for locomotion.

TypesThe three most common types of mollusks are:● gastropods● bivalves● cephalopods

Gastropoda

● Gastropods make up 80% of Molluscs which consist of about 37,500 living species.

● Most gastropods have an external shell.● 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.

● The most common gastropods are:○ Snails - land and aquatic○ Slugs○ Nudibranchs○ Pteropods - Thecosomata - sea butterflies

Bivalvia

● Also known as Pelycopoda or Lam ellibranchia● Bivalves always have two shells held together by hinges

and strong muscles.● Some are omnivores (an animal or person that eats food

of both plant and animal origin) and also eat bacteria and protists.

● They filter feed in which food sticks to the mucus on their gill, then cilia move to their mouth.

● They may live in any watery environment because they are able to swim and float around.

● They are the only type of molluscs that does not have a radula.

● Bivalves lack defined heads.● Eyes may be present somewhere else on the body.● They have an open circulatory system.● They also have sensory cells.● There are approximately 7,500 living species of bivalves.● The name bivalves derives from their two valves, their

two shells in which it is divided. ● Most common bivalves include:

● Mussels● Clams● Oysters● Scallops

Cephalopoda

● Cephalopods either have internal shells, external shells, or no shell at all.

● They are carnivores.● They capture prey with their muscular tentacles,

crush it with their beak, and scrape the flesh off with their radula.

● There are about 600 living species.

● They all live in the ocean.● They are able to move through jet propulsion

(squeezing a current of water out of their mantle cavity and through a tube).

● Their closed circulatory system makes them the fastest, smartest, and biggest of all molluscs.

● Some common cephalopods include:○ Octopus○ Squid○ Cuttlefish○ Nautilus

Importance of Molluscs● Molluscs serve as food for humans as well as

for other animals.● They are used for pearls and Native

American currencies.● Some are used as environmental indicators

because their bodies are filled with water.● There are about 85,000 species of molluscs.● It is the largest marine phylum.● There is a risk of food poisoning from toxins

that accumulate in moluscs.● Their bites and stings are fatal and some are

venomous.