PHYLUM MOLLUSCA; General Characteristics and Classes.


PHYLUM MOLLUSCA; General Characteristics and Classes.


Mollusca is the largest lophotrochozoan phylum and one of the largest and most diverse of all phyla. Molluscs range in size from microscopic organisms to the largest of invertebrates.

Most molluscs are marine, but some are freshwater, and a few are terrestrial. They occupy a wide variety of niches.

Many molluscs are economically important, and a few are medically important as hosts of parasites.

Ocean acidification is a serious threat to molluscs because increased acidity makes it harder to secrete calcium, the key component in healthy shells. Bivalves, such as oysters, are particularly affected, damaging some local economies.

Mollusca is one of the largest animal phyla after Arthropoda.

Kingdom Fungi, Phyla, Classification and characteristics.

Life process of bony fish (Osteichthyes “Tilapia”)

Life Process of a Butterfly (Citrous swallowtail butterfly)

Moss and Fern.

General features and function.

  • The basic body divisions of molluscs are head-foot and visceral mass, which is usually covered by a shell.
  • The foot is usually a ventral, sole-like, locomotory organ, but it may be variously modified, as in cephalo- pods, where it has become arms and a funnel.
  • The mantle secretes the shell and overlies a part of the visceral mass to form a cavity housing the gills. The mantle cavity has been modified into a lung in some.
  • The radula is a protrusible, tongue-like organ with teeth used in feeding. It occurs in all molluscs except bivalves and many solenogasters.
  • The circulatory system of molluscs is open, with a heart and blood sinuses, except in cephalopods, which have a closed circulatory system.
  • The primary larva of molluscs is the trochophore; in most marine molluscs, the trochophore develops into a second larval stage, the veliger.
  • Most molluscs have a complex nervous system with a variety of sense organs. ∙     Molluscs typically have a pair of nephridia connecting to the coelom.
  • Molluscs are coelomate, although their coelom is limited to the area around the heart, the gonads, and occasionally part of the intestine.
  • In many animals, the coelom cushions and protects the visceral organs and may serve as a hydrostatic skeleton for locomotion.
  • However, the hard, external shell of most molluscs precludes use of the coelom for cushioning, shape change, or locomotion.

Characteristics of Organism in Phylum Mollusca

  • Dorsal body wall forms pair of folds called the mantle, which encloses the mantle cavity, is modified into gills or lungs, and secretes the shell (shell absent in some); ventral body wall specialized as a muscular foot, variously modified but used chiefly for locomotion; radula in mouth
  • Free-living or occasionally parasitic
  • Live in marine, freshwater, and terrestrial habitats
  • Body bilaterally symmetrical (bilateral asymmetry in some); unsegmented; often with definite head
  • Triploblastic body
  • Coelom limited mainly to area around heart, and perhaps lumen of gonads, part of kidneys, and occasionally part of the intestine
  • Surface epithelium usually ciliated and bearing mucous glands and sensory nerve endings
  • Complex digestive system; rasping organ (radula) usually present; anus usually emptying into mantle cavity; internal and external ciliary tracts often of great functional importance
  • Circular, diagonal, and longitudinal muscles in the body wall; mantle and foot highly muscular in some classes (for example, cephalopods and gastropods).
  • Nervous system of paired cerebral, pleural, pedal, and visceral ganglia, with nerve cords and subepidermal plexus; ganglia centralized in nerve ring in gastropods and cephalopods
  • Sensory organs of touch, smell, taste, equilibrium, and vision (in some); the highly developed direct eye (photosensitive cells in retina face light source) of cephalopods is similar to the indirect eye (photosensitive cells face away from light source) of vertebrates but arises as a skin derivative in contrast to the brain eye of vertebrates
  • Both monoecious and dioecious forms; spiral cleavage; ancestral larva a trochophore, many with a veliger larva, some with direct development.
  • One or two kidneys (metanephridia) opening into the pericardial cavity and usually emptying into the mantle cavity.
  • Gaseous exchange by gills, lungs, mantle, or body surface
  • Open circulatory system (secondarily closed in cephalopods) of heart (usually three chambered), blood vessels, and sinuses; respiratory pigments in blood
  • No asexual reproduction


Class Caudofoveata

  • They are mostly burrowers and orient them- selves vertically, with the terminal mantle cavity and gills at the entrance of the burrow.
  • They feed primarily on microorganisms and detritus.
  • They are wormlike.
  • Shell, head, and excretory organs absent.
  • Radula usually present
  • Mantle with chitinous cuticle and calcareous sclerites
  • Oral pedal shield near anterior mouth.
  • Mantle cavity at posterior end with pair of gills.
  • Sexes separate.
  • Formerly united with solenogasters in class Aplacophora.
  • This class is sometimes called Chaetodermomorpha.
  • Examples: Chaetoderma, Limifossor.

Class Solenogastres

  • They are also wormlike.
  • Shell, head, and excretory organs absent.
  • Radula may be present or absent.
  • Mantle usually covered with calcareous sclerites or spicules.
  • Rudimentary mantle cavity posterior
  • They are without true gills but sometimes with secondary respiratory structures
  • Foot represented by long, narrow, ventral pedal groove
  • And they hermaphroditic.
  • Solenogasters are bottom-dwellers, and often live and feed on cnidarians.
  • This class is sometimes called Neomeniomorpha.
  • Example: Neomenia.

Class Polyplacophora

  • Elongated, dorsoventrally flattened body with reduced head
  • Their head and cephalic sensory organs are reduced, but photo- sensitive structures (esthetes), which have the form of eyes in some chitons, pierce the plates.
  • Bilaterally symmetrical
  • Radula present
  • Shell of seven or eight dorsal plates
  • Foot broad and flat
  • Gills multiple along sides of body between foot and mantle edge
  • Sexes usually separate with a trochophore but no veliger larva.
  • They prefer rocky surfaces in intertidal regions, although some live at great depths.
  • A pair of osphradia (chemoreceptive sense organs for sampling water) occupy the mantle grooves near the anus of many chitons.
  • Examples: Mopalia

Class Monoplacophora

  • Body bilaterally symmetrical with a broad flat foot
  • A single limpetlike shell
  • Mantle cavity with three to six pairs of gills
  • Large coelomic cavities.
  • Radula present
  • Three to seven pairs of nephridia two of which are gonoducts
  • Separate sexes.
  • Example: Neopilina

Class Gastropoda

  • Class Gastropoda comprises the largest and most diverse group of molluscs.
  • All gastropods exhibit torsion, a stage in development where the anus is directly over the head.
  • Most have coiling, an elongation and spiralling of the visceral mass.
  • Torsion results in fouling, the release of digestive waste over the head and in front of the gills.
  • Among the solutions to fouling are bringing water into one side of the mantle cavity and carrying waste out via the other side (abalone, limpets, many others), some degree of detorsion (opisthobranchs), and conversion of the mantle cavity into a lung (pulmonates).

Body asymmetrical and shows effects of torsion;

The body is usually in a coiled shell (shell uncoiled or absent in some)

Head well developed,

Have radula; foot large and flat

One or two gills, or with mantle modified into secondary gills or a lung most with single atrium and single nephridium

Nervous system with cerebral, pleural, pedal, and visceral ganglia

Dioecious or monoecious, some with trochophore, typically with veliger, some without pelagic larva.

Examples: Busycon, Polinices, Physa, Helix, Aplysia

Major Groups of Gastropods

Traditional taxonomy of class Gastropoda recognizes three sub-classes:

  • Prosobranchia, the largest subclass, almost all of which are marine;
  • Opisthobranchia, an assemblage including sea slugs, sea hares, nudibranchs, and canoe shells, all marine;
  • And Pulmonata, containing most freshwater and terrestrial species.

Currently, gastropod taxonomy is in flux.

Evidence suggests that Prosobranchia is paraphyletic.

Opisthobranchia may be paraphyletic, but Opisthobranchia and Pulmonata together apparently form a monophyletic grouping.

The number of subclasses within the Gastropoda and the relationships among them remain subjects of considerable controversy.

Class Bivalvia

In members of Class Bivalvia, the shell is divided into two valves joined by a dorsal ligament and held together by an adductor muscle.

Most are marine or freshwater filter feeders, drawing water through their gills by ciliary action.

  • A radula is not present.
  • Body enclosed in a two-lobed mantle
  • Shell of two lateral valves of variable size and form, with dorsal hinge
  • Head greatly reduced, but mouth with labial palps.
  • No cephalic eyes, a few with eyes on mantle margin
  • Foot usually wedges shaped
  • Gills platelike
  • Sexes usually separate, typically with trochophore and veliger larvae.
  • Examples: Anodonta, Venus, Tagelus, and Teredo

Class Scaphopoda

Scaphopoda is a small class whose members have a tubular shell, open at both ends, and the mantle wrapped around the body.

Body enclosed in a one-piece tubular shell open at both ends;

Conical foot; mouth with radula and contractile tentacles (captacula);

Head absent and mantle for respiration;

Sexes separate with trochophore larva.

Example: Dentalium

Class Cephalopoda

Members of class Cephalopoda are all predators and many can swim rapidly. Their arms or tentacles, derivatives of the foot, capture prey by adhesive secretions or by suckers. They swim by forcefully expelling water from their mantle cavity through a funnel.

Shell often reduced or absent;

Head well developed with eyes and a radula

Head with arms or tentacles

Foot modified into siphon

Nervous system of well-developed ganglia centralized to form a brain

Sexes separate with direct development.

Examples: Sepioteuthis, Octopus, Sepia, squids, cuttlefish, and nautilus.

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