MOLLUSCA

PHYLUM MOLLUSCA

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by: Geonyzl J. Lepiten

Mollusca

- from the Latin word “Mollis” meaning soft. (From the Latin Molluscus meaning

soft of body) - first used by Curvier in 1798. - one of the most successful groups

in the animal kingdom. - found in all habitats - divided into 8 classes:

- Class Gastropoda - Class Pelecypoda (Bivalva) - Class Cephalopoda - Class Aplacophora - Class Monoplacophora - Class Polyplacophora (Chitons) - Class Scaphopoda (Tusk Shells) - Class Cuadofoveata

Anatomical features:

Flexible body wall, that surrounds the body.

It varies from some species to species , some part of the wall were folded to form a MANTLE.

In some species, part of the wall formed into a SHELL

On the underside the wall is stretched to form a thickened mass called the foot.

A. The Body Wall

Some part of the wall was used for locomotion, food entrapment, and prevention of water loss.

Muscle tissue is present (but much more found in the foot since it is used for locomotion).

B. The Shell

It is formed by secretions of glandular cells in the mantle.

The thin outer layer is known as periostracum which is composed of organic layer.

A thin innermost calcareous layer known as nacreous layer (secreted by the edge of the mantle).

An thick calcareous middle layer, known as prismatic layer.

Digestive tract

It is complex and complete.

The foregut region consist of an esophagus, stomach and mouth.

The mouth cavity is surrounded by toothed belt known as RADULA.

The pattern of the stomach varies according to the mollusks diet.

Radula is used for scrapping algae.

Is through the gills called CTENIDIA. (located in mantle cavity)

Excretion of waste through the metanephridia, gills and body.

But terrestrial species have lost their ctenidia and replaced by lungs that can function for both water and air.

These would vary from species to species.

The Respiration and Excretion

The Circulatory and Nervous System

Blood circulates through the gill filaments.

Most mollusk blood have a respiratory pigment known as HEMOCYANIN ( copper compound ).

The oxygenated blood is bluish in color, while deoxygenated blood is colorless.

Blood circulation would vary and it is mediated by a muscular heart.

It also possess well-developed sensory organs. The highest degree of development are found in CLASS CEPHALOPODA

Reproduction Reproduction is sexual.

The fertilized egg develops into a swimming form called TROCHOPHORE larva.

The larva stage is also present in the development of the annelids.

Then elongates to become a VELIGER LARVA.

Class Gastropoda

From the Greek word “Gastro + poda” means stomach foot.

DEFINING CHARATERISTICS:

Visceral mass nervous systems become twisted 90 – 180 degrees (exhibiting torsion)

Proteinaceous shield on the foot.

(Class Gastropoda…….)

Most of the approximately 40,000 living species of gastropods have shells.

However there are quite a few groups that have either reduced or internal shells, or no shell at all.

Shelled forms are generally called "snails "

Forms without shells are called "slugs“

However the terrestrial slugs are not closely related to the various marine forms that are without shells.

Although most Gastropods are marine, there are numerous forms in both freshwater and terrestrial environments

ANATOMY OF THE GASTROPOD WITH SHELL

Class Pelecypoda (Bivalva)

The bivalves include forms such as clams , mussels , scallops , oysters , as well as many less familiar forms.

Most bivalves are ciliary feeders (or filter-feeders)

Bivalves are exclusively aquatic;

Bivalves are laterally compressed animals, with two shell "valves " that are hinged on the animal's dorsal surface.

The approximately 8,000 living species of bivalves are mostly marine, but there are many freshwater species as well (however, no terrestrial ones

Continue on Bivalva……

Bivalves are found in just about every marine environment, from the intertidal zone to the deepest marine habitats.

Spondylus americanus Glycymeris glycymeris Felaniella vilardeboana

Most bivalves are suspension feeders, filtering small organisms and organic particles from the water (such as bacteria , phytoplankton , zooplankton , and nonliving organic detritus ).

Left Valve and Dorsal View of a Clam Shellafter Brusca & Brusca 1990

Bivalve continue…

The shell is made up of three layers: 

The periostracum or thin outer layer that is made of horny, organic material called conchiolin,

the prismatic or thick middle layer that is made up of calcium carbonate crystals arranged in vertically,

and the nacerous or thinner inner layer that is composed of thin horizontally arranged calcium carbonate crystals

Bivalve continue……

Food entering the mouth is passed anteriorly to the stomach via ciliary action.

  The stomach is surrounded by a large digestive gland and is divided into two regions.  In the first region (dorsal) the esophagus and ducts of the digestive gland enter and it contains a ventral style sac.

Food and nutrition:

This dorsal portion of the stomach is lined with chitin except for the large folded and ciliated sorting region, into which the digestive gland opens. 

At the apex of the stomach is a tooth-like projection called the gastric shield, which arises from the chitinous girdle.  At the end of this region is the cecum.

Members of the Class Bivalva

A scallop Tridacna (Tridacna)

gigas

Pharella javanica

Class Cephalopoda

Cephalopods , which include the familiar squids and octopus include species which are the largest known invertebrates (giant squid , up to 20 meters long, including tentacles),

The most intelligent, and the fastest swimming aquatic invertebrates (squid ).

There are also forms with external shells (nautilus ), and internal hard shells (cuttlefish ).

Most of the approximately 650 living species of Cephalopods are active swimmers

(continue on Class Cephalopods……)

Cephalopods have a closed circulatory system, an adaptation to their active lifestyle, as opposed to the open circulatory system found in other molluscs .

Squid in particular are often very abundant in pelagic marine environments, where they are voracious predators of many organisms, especially fish.

In return, they are also the major prey item of many fish and some marine mammal species.

Anatomy of the Cephalopods

Class Aplacophora

250 living species (poorly studied!) of marine, wormlike, bilaterally symmetrical living at moderate, to very great depths, usually on or in soft bottoms.  They have no shell, but have calcareous spicules in the body surface. 

The foot is restricted to an anterior pedal shield or to a narrow groove running the length of the body.

Aplacophorans have a radula and a posterior mantle cavity. 

Some are detritus feeders, others are predators. 

They range in length from 1 to 300 millimeters.

Summary

Characteristics of Mollusca:-

Bilaterally symmetrical.

Body has more than two cell layers, tissues and organs.

Body without cavity.

Body possesses a through gut with mouth and anus.

Body monomeric and highly variable in form, may possess a dorsal or lateral shells of protein and clacareous spicules.

Has a nervous system with a circum-esophageal ring, ganglia and paired nerve chords.

Has a true closed circulatory system with a heart.

Has gaseous exchange organs called ctenidial gills.

Continue……

Has a pair of kidneys.

Reproduction normally sexual and gonochoristic.

Feed a wide range of material.

Live in most environments