Chapter 27-2 PHYLUM MOLLUSKA

Mollusks

THE WORD MOLLUSK MEANS “SOFT BODY” MOLLUSKS ARE SOFT BODIED INVERTEBRATES.

But these are hard shells.!!!!*** Where’s the “beef”???

ANIMAL CLASSIFICATION

CHARACTERISTICS OF MOLLUSKS

1. SOFT BODY COVERED BY A FOLD OF TISSUE CALLED A MANTLE 2. HEAD AND A MUSCULAR FOOT FOR MOVEMENT

4 Characteristics of Mollusks

All Mollusks have: 1. Muscular foot - used for locomotion 2. Mantle – thin, delicate tissue that covers the body of the mollusk, like a cloak 3. Shell – usually CaCO 3 , and secreted by mantle 4. Visceral mass – just below mantle, containing the internal organs

Foot of a Clam

Foot, Shell, Mantle, Visceral Mass are always Present in Mollusks

MOLLUSKS ARE GROUPED

INTO 3 CLASSES ACCORDING TO THE TYPE OF FOOT SHELL / NO SHELL KIND OF SHELL

2 CLASSES WITH SHELLS OUTSIDE THE BODY

UNIVALVE

(ONE SHELLED MOLLUSK)

BIVALVE

(TWO SHELLED MOLLUSK)

3RD CLASS HAS NO SHELL

•INSIDE •OR •OUT

OCTOPUS

SQUID

The All Important

COELOM

is alive and well in the Mollusks. Here’s why it’s important… The development of a coelom allows for the formation of more complex tissues and organs.

It also allowed for a wide variety of body architectures; which allows for coelomates to grow larger.

The primary means of locomotion is via a muscular

FOOT

.

Sometimes the foot is modified, but it is always present.

Mollusks have a definite “head” region that contains organs for chemo-sensory functions.

All mollusks, except bivalves, possess a rasping tongue organ called the

RADULA

.

Anatomy of the Rasping Tongue, or Radula of a Snail

Function of the Radula

• A layer of flexible skin, with hundred of sandpapery teeth, used to scrape algae off of rocks • In full-on carnivore mode, the radula acts like a drill to pierce shells. These things even have poison glands to make things nastier  Octopi and certain sea slugs do this.

Bivalves are Usually Filter Feeders • Think “Clam” when Bleecker says Bivalve.

• Come on, say it out loud now  • Feathery Gills are extended from Sessile, or virtually motionless bivalves to screen the water for microorganisms and algae to munch on

Bleecker Swam up to one of these in Hanauma Bay, on Oahu (Hawaii, people!)

Respiration

• Gills are used, and have many capillaries to permit diffusion of O 2 into blood stream • Gills on the Nudibranch are particularly cool 

Circulatory System

• Open Circulatory System – the blood does not always travel in blood vessels, and often deposits into spaces or sinuses to drain back once organs have been bathed with blood • This works for clams and snails, but not highly motile octopi or squid, which have Closed Circulatory Systems

Excretion- Waste Management 101 • 2 openings in the digestive tract • Ammonia wastes from protein digestion can be toxic, and are eliminated via Nephridia, simple tube shaped organs leading out of the body, via the skin.

Reproduction

• In a watery environment, egg and sperm are often released in mass quantities in hopes of fertilization. Larvae then swim to a spot on the bottom and grow into bivalves, etc.

• In tentacled mollusks, modified tentacles are used in Internal Fertilization.

• Spontaneous sex changes do occur, if there are not enough members of a given sex. Holy cross dressing Batman!

Classifying Mollusks Further

1. Class Gastropoda – stomach foot – Broad muscular foot on stomach side – Snails, Slugs, Nudibranchs – Some shelled, others not – Snail Shell shows RADIAL TORSION, creating the distinct spiral shape

Two Shelled Mollusks – Class Bivalvia Oysters, Scallops, Clams, Mussels Free swimming larvae Mantle manufactures the shell Pearls occur when a sand grain or foreign particle is covered in “mother of pearl”, a material used to keep the inside of the shell smooth

The Pearl Emerges – Black is Particularly Rare 

Classifying Mollusks Further

2. Class Cephalopoda – “head foot” – High degree of CEPHALIZATION – Octopi, Squid, Nautilus, Cuttlefish – Tentacled and highly motile, with excellent senses, particularly the eyes – Originated 500 million years ago, in Cambrian Period – No shell, but internal shell is present – Jet propulsion – Ink defense mechanism, Poisonous radula – ewww!

Bleecker-Vision!

The Nautilus

Bioluminescence! Complicated Chemical Reactions!

Ecological Role

1. Food source – clams, oysters, squid, octopi 2. Filter feeders remove pollutants 3. Indicators of pollution 4. Ships worn down by bivalves 5. Recyclers of waste or detritus