Phylum Mollusca – Definition, Characteristics, Classification, Examples

Definition

The members of Phylum Mollusca are triploblastic, bilaterally symmetrical, eumetazoan, whose body organization is of organ system grade, but are unsegmented.

The body is soft, covered with a mantle and usually with a calcareous shell. They are mostly aquatic, but however, some are terrestrial or semiaquatic.

There are some 80,000 species of living molluscs known to us. The study of Mollusca and their shells is known as Conchology or Malacology.

Brief History

Molluscs are known to man since ancient times. Johnston (1650) proposed the name Mollusca for the first time to include unsegmented animals covered with mantle.

Characteristics Features of Mollusca

  1. The members of Phylum-Mollusca (Lat., Mollis = Soft body) are soft-bodied, unsegmented, triploblastic metazoans without jointed appendages.
  2. Bilateral symmetry secondarily lost in gastropods and cephalopods.
  3. Shell is external or internal and is made up of calcium carbonate.
  4. The body consists of the head, foot and visceral mass.
  5. The characteristic ventral foot is present for different purposes of creeping and ploughing.
  6. Body covered with a mantle.
  7. Gills or ctenidia and mantle cavity the organs of respiration.
  8. A chitinous, ribbon-shaped radula is present in the oral cavity in some.
  9. The alimentary canal is complete. A large liver and salivary glands are present.
  10. The circulatory system is open type and consists of a dorsal heart having one or two auricles and one ventricle. Anterior aorta is present.
  11. Three important pairs of ganglia-(i) cerebral, (ii) pedal, and (iii) pleural are present. These are connected by circumoesophageal ring.
  12. An olfactory organ or osphradium is present at the base of gills.
  13. One or two pairs of kidneys for excretion.
  14. Sexes are separate, gonads are one or two.
  15. Three larval stages are present-(i) Trochosphere, (ii) Veliger, and (iii) Glochidium.
  16. Examples of Mollusca are: Chiton, Snail, Mussel, Dentalium, Sepia, Octopus etc

Classification of Phylum Mollusca

Classification of Mollusca
Classification of Mollusca

Class 1-Amphineura

  • Body elongated, dorsoventrally flattened head reduced without tentacles and eyes.
  • Radula present.
  • Shell is made up of 8 plates or absent.
  • Nerve cord double.
  • Foot ventral, large, adapted for adhering rocks or absent.

Order 1-Polyplacophora

  • Body dorsoventrally flattened.
  • Radula, mantle, external gills are present.
  • Shell is made up of 8 pieces.
  • Foot flat and ventral.
  • Example-Chiton.

Order 2—Aplacophora

  • 1. Body worm-like and cylindrical, without shell and foot.
  • Radula simple, mantle present.
  • Calcareous spicules buried in the cuticle.
  • Gills present.
  • Example-Chaetoderma.

Class 2-Scaphopoda

  • Body enclosed in a tubular shell, open at both ends.
  • Tusk shells.
  • Head absent, eyes absent, mouth with tentacles.
  • Foot conical, gills absent.
  • Kidney paired, gonad single.
  • Dioecious.
  • Example-Dentalium.

Class 3-Pelecypoda (Bivalvia)

  • Body laterally compressed.
  • Body enclosed in a bivalved shell.
  • Head absent, tentacles, eyes, jaws, radula are absent.
  • Foot well-developed and wedge-shaped.
  • The filter-feeding mechanism is found.
  • Dioecious forms, veliger larva is present.
  • Marine or fresh-water forms.

Order 1– Protobranchia

  • Single pair or plume-like ctenidia.
  • Mouth at the base of the muscular proboscis.
  • Stomach with style-sacs.
  • The foot is ventral, flattened, sole-like, used for creeping.
  • Example-Nucula.

Order 2- Filibranchia

  • Single pair of plate-like ‘V’-shaped gills.
  • Chitinous gastric shield in the stomach.
  • Style-sac with crystalline style.
  • Foot small or poorly developed.
  • Example-Mytilus, Ostrea.

Order 3-Eulamellibranchiata

  • 1. Gills are firm and basket-like.
  • Foot large, byssus small or absent.
  • The foot is long and slender.
  • Example-Unio, Teredo.

Order 4-Septibranchia

  • Gills reduced.
  • The Stomach is lined by chitin, style-sac reduced.
  • Foot long and slender.
  • Marine forms.
  • Example-Poromya.

Class 4-Gastropoda

  1. It includes snails and slugs.
  2. The Head is well-developed, eyes and tentacles present.
  3. Foot large and flat, sole-like radula is present.
  4. Shell, when present, is spirally coiled.
  5. Torsion of visceral mass during the development.
  6. Marine and fresh-water forms.

Subclass 1-Prosobranchia

  • Body mass shows torsion.
  • Head with a single pair of tentacles and a pair of eyes.
  • Two gills placed anteriorly to the heart.
  • Shell spirally coiled and the opening is closed by an operculum.
  • The nervous system is twisted in the form of ‘8’.
  • Sexes are separate.

Order 1-Aspidobranchia

  • One or two bipectinate ctenidia.
  • Two kidneys, heart with two auricles.
  • Gonads convey the contents through the kidney.
  • Examples-Patella, Trochus.

Order 2-Pectinibranchia

  • One monopectinate gill.
  • Siphon operculum and penis are present.
  • Nervous system with pedal cords.
  • Shell is coiled.
  • Fertilization internal, veliger larva present.
  • Examples-Pila, Cypraea.

Order 3-Stenoglossa

  • Shell with a long siphonal canal.
  • The nervous system concentrated.
  • Ospharidrum is large.
  • Larva suppressed.
  • Examples: Murex, Buccinium.

Subclass 2-Opisthobranchia

  • 1. Shell small, without operculum or very much reduced.
  • Body mass torted or detorted.
  • Gills posterior to heart, replaced by secondary branchiae or gills.
  • Nervous system not twisted.
  • Monoecious, veliger larva present.
  • Totally marine.

Order 1-Cephalaspidea

  • 1. Shell moderately developed.
  • Head with a tentacular shield.
  • Lateral parapodial lobes prominent. Example-Bulla.

Order 2-Anaspidea

  • Shell usually reduced or internal, covered by the mantle.
  • Well-developed parapodial lobes.
  • Head with a pair of rhinophores, tentacles and a pair of eyes. pair of
  • Example-Aplysia.

Order 3-Pteropoda

  • Shell present or absent.
  • Parapodial fins for swimming.
  • With or without mantle cavity.
  • Pelagic forms.
  • Example-Cavolina.

Order 4-Sacoglossa

  • Shell present or absent.
  • Pharynx suctorial.
  • Example-Oxynoe.

Order 5-Acochilidiacea

  • Minute forms, with or without a shell.
  • Visceral mass separated from foot, spines present.
  • Live in coarse sand.
  • Example-Acochlaium.

Order 6-Notaspidea

  • Shell external, reduced or internal.
  • Mantle without mantle cavity.
  • Bipectinate osphradium on right side.
  • Parapodia absent. Example-Pleurobranchus.

Order 7-Nudibranchia

  • Shell, gill, mantle cavity and osphradium absent.
  • Respiration by secondary branchiae.
  • Example-Doris.

Order 8-Pyramidellacea

  • Shell typically spirally coiled.
  • Operculum present, but gill and radula absent.
  • Long invaginable proboscis present.
  • Semiparasitic forms.
  • Example-Pyramidella.

Order 9-Philinoglossacea

  • Small, naked snails, without gill and head.
  • Visceral mass separated only by a groove.
  • Examples Philinoglossa, Sapha.

Order 10-Rhodopacea

  • Vermiform snails, without external appendages.
  • Anus on the right side of the body.
  • Example-Rhodope.

Order 11-Onchidiacea

  • Slug-like, without shell, pulmonary sac.
  • Head with a pair of tentacles tipped with eyes.
  • Posteriorly located anus.
  • Example-Oncidium.

Order 12-Parasita

  • Endoparasite in holothurians.
  • Extremely degenerated snails.
  • Example-Entoconcha.

Subclass 3-Pulmonata

  • Detorted body mass.
  • With or without shell.
  • Operculum and gills absent.
  • Pulmonary sac with a pore on right side.
  • Hermaphrodite, fresh-water forms.

Order 1-Basommatophora

  • Shell delicate with conical shape.
  • One pair of non-invaginable tentacles.
  • Eyes at bases of tentacles.
  • Male or female pore separate.
  • Examples Siphonaria, Planorbis.

Order 2-Stylommatophora

  • Shell with the conical shape or internal or absent.
  • Two pairs of invaginable tentacles.
  • Second pair of tentacles with eyes at their tips.
  • Male and female genital pores united.
  • Examples Helix, Limax.

Class 5-Cephalopoda

  • Body elongated, divisible into head and trunk.
  • Shell external or internal or absent.
  • Head distinct with large eyes and mouth.
  • Foot modified into arms and funnel, surrounding the head.
  • Dioecious, direct development.
  • Marine, free-swimming forms.

Subclass 1-Nautiloidea (Tetrabranchia)

  • Shell external, coiled, without sutures.
  • Recept species with many suckerless tentacles.
  • Two pairs of gills, two pairs of nephridia.
  • Chamber joined by siphuncle.
  • Eyes simple.
  • Example-Nautilus.

Subclass 2-Ammonoidea

  • Extinct species.
  • Shell external and coiled, with complex septa and sutures.
  • Example-Pachydiscus.

Subclass 3-Coleoidea (Dibranchia)

  • Body elongated with lateral fins.
  • Shell internal or absent.
  • Foot modified into tentacles and suckers.
  • One pair of gills, one pair of nephridia.

Order 1-Decapoda

  • Ten arms, two elongated to form the tentacles and the rest eight are short arms.
  • Ink gland generally present.
  • Fins present.
  • Examples Sepia, Loligo.

Order 2-Octopoda

  • Body globular.
  • Fins absent.
  • Eight equal arms surround the head.
  • Example-Octopus.

Class 6-Monoplacophora

  • Body bilaterally symmetrical with a dome-shaped mantle.
  • Flattened limpet-shaped mantle.
  • Univalve shell.
  • Head with eyes and tentacles.
  • Foot broad and flat, with 8 pairs of pedal retractor muscles.
  • Five pairs of gills in the pallial groove.
  • Six pairs of nephridia, two of which are gonoducts.
  • Radula in radular sac, intestine much coiled.
  • Heart with two pairs of auricles and a single ventricle.
  • Nervous system with longitudinal, pallial and pedal cords.
  • Internal segmentation.
  • Marine and primitive mollusc.
  • Example-Neophilina galathea.

Some Examples of Mollusca

CHITON

Classification of Chiton

PhylumMollusca
ClassAmphineura
OrderPolyplacophora
GenusChiton

Distribution: Cosmopolitan in distribution, found mainly in shallow water. Few species also live in deep sea.

Habits and Habitat:

  1. Chiton is a marine animal and creeps slowly for very small distance.
  2. It is nocturnal and herbivorous.
  3. When at rest, it adheres firmly to rocks or corals by means of sucker-like foot.

Characteristics Feature of Chiton

  1. It is commonly known as Sea Mouse.
  2. Chiton is marine and sluggish animal, usually found attached to rock, empty shell etc.
  3. Body is elliptical, bilaterally symmetrical and dorsoventrally flattened, measuring 3 to 5 cm. in length.
  4. Body consists of small head, a large flat foot and a dorsal mantle forming a roof-like covering.
  5. The dorsal side of mantle contains a linear series of 8 calcareous overlapping plates called shell plates helping animal to roll.
  6. Eyes and tentacles are absent.
  7. Mouth and anus are placed at the opposite ends.
  8. Several pairs of bipectinate ctenidia are found in the mantle grooves.
  9. On the ventral side, a large creeping sole or foot, mouth, labial palps, ctenidia, pallial groove and anus are seen.
  10. Sexes are separate, fertilization internal, larva is called trochophore.
  11. Chiton is used as food.
Chiton (A) Dorsal View (B) Ventral View

PILA

Classification of Pila

PhylumMollusca
ClassGastropoda
OrderPectinibranchia
GenusPila

Distribution: Commonly found in India, Europe and U.S.A.

Habits and Habitat:

  1. It is the largest freshwater mollusc found in ponds, tanks, streams and rice plantations.
  2. It is mainly herbivorous and feeds upon succulent water plants. Occasionally it has been found feeding on dead animals tissue.

Characteristics Feature of Pila

  1. It is commonly known as Apple snail.
  2. It is amphibious, i.e., it can live on land and in water both. It hibernates in winter in mud.
  3. Body is covered by a thick yellow-coloured or brown globular univalve shell, comprising body whorl, penultimate whorl and apex. Each division is separated by sutures.
  4. Lines of growth called varices are visible on the shell.
  5. The shell is spirally coiled around an axis called columella and opens to the exterior by mouth or oral aperture.
  6. Body is divisible into head, visceral mass and foot.
  7. Operculum well-developed and closes the mouth aperture.
  8. Foot is well-developed and is a creeping organ.
  9. Head lies on the upper side and bears two pairs of tentacles and a pair of eyes.
  10. Visceral mass consists of main organs of the body which are spirally coiled, covered by mantle.
  11. Supposed to have medicinal value.
  12. Sexes are separate, sexual dimorphism is not well-marked.
  13. Fertilization is internal, development is direct.
Pila Labeled Diagram

APLYSIA

Classification of Aplysia

PhylumMollusca
ClassAmphineura
OrderOpisthobranchia
GenusAplysia

Distribution: Found in India, Asia, West Indies and Florida.

Habits and Habitat:

  • Marine, found up to a depth of 40 fathoms, herbivorous, sluggish, is found slowly crawling among seaweeds.

Characteristics Feature of Aplysia

  1. Commonly known as Sea Hare.
  2. The body is soft and fleshy, showing greenish tinge.
  3. The anterior end of the body bears head and neck.
  4. Head bears a pair of anterior tentacles and a pair of posterior tentacles or rhinophores.
  5. A pair of eyes is present at the base of tentacles.
  6. The mantle cavity is open on right side with a backwardly direction ctenidium.
  7. Visceral mass raised into a prominent hump.
  8. The root is muscular, elongated and pointed posteriorly.
  9. A pair of fleshy lateral outgrowths of foot called parapodia are used as fins for swimming.
  10. Shell in internal and reduced to a thin flexible plate.
  11. Animal is bisexual with a single gonoduct and a genital aperture.
  12. Aplysia escapes the observation of enemies by secreting a purple fluid from the glands.
Aplysia Labeled Diagram

UNIO

Classification of Unio

PhylumMollusca
ClassBivalvia (Pelecypoda)
OrderEulamellibranchiata
GenusUnio

Distribution: Commonly found in India.

Habits and Habitat:

  1. It is Indian mussel found in freshwater streams, rivers and lakes.
  2. Slow-moving, feeds on microscopic plants and animals.

Characteristics Feature of Unio

  1. Commonly known as fresh-water mussel or calm.
  2. The body is unsegmented, bilaterally symmetrical and flattened from side to side, measuring 5-10 cms. in length.
  3. The body is completely enclosed in a pair of bivalve shells.
  4. Both the valves are united by hinge-ligament.
  5. A whitish, knob-like swelling known as umbo is present anteriorly on each shell valve.
  6. Lines of growth are visible on shell.
  7. Exhalant siphons and inhalant siphons are present posteriorly on the margins.
  8. Paired bipinnate gills present on either side of visceral mass.
  9. Foot is large, muscular and wedge-shaped used for burrowing.
  10. Adductor muscles close the shell.
  11. Sexes are separate, sexual dimorphism absent.
  12. Fertilization internal, development indirect through glochidium larva.
  13. Mussels and oysters are used as food.
Unio Labeled Diagram

OCTOPUS

Classification of Octopus

PhylumMollusca
ClassCephalopoda
OrderOctopoda
GenusOctopus

Distribution: Cosmopolitan, specially found in India, Europe, Atlantic and Pacific coasts.

Habits and Habitat:

  1. Marine, deep sea animals, nocturnal in habit.
  2. It constructs a den or shelter of stones to which it always returns after excursion in search of food which includes crabs, fishes and other molluscs.
  3. It is a carnivorous and voracious feeder.

Characteristics Feature of Octopus

  1. Commonly known as Devil fish due to its ghostly appearance.
  2. The body is round and differentiated into head and visceral hump.
  3. Head contains eyes, siphon and 8 elongated arms. Each arm is provided with a double row of sessile suckers.
  4. Two arms get changed into intromittent organ called hectocotylised arms.
  5. The shell is absent.
  6. Giant octopus can attack man.
  7. It ejects an inky fluid for protection forming a screen around it.
Octopus Diagram

SEPIA

Classification of Sepia

PhylumMollusca
ClassCephalopoda
OrderDecapoda
GenusSepia

Distribution: Cosmopolitan but mainly found in coastal water in India, Europe and Mediterranean region.

Habits and Habitat:

  1. It is a marine mollusc, lives usually at a depth of a few fathoms but often comes into shallow water.
  2. Active swimmer and carnivorous feeds upon crabs, prawns and other crustaceans.

Characteristics Feature of Sepia

  1. Commonly known as Cuttle fish.
  2. Body is fleshy, brown in colour and differentiated into visceral hump. head and
  3. Head contains the mouth, eyes and eight small arms and two long tentacles (modified foot). Arms and tentacles are provided with suckers.
  4. The visceral hump is thick, fleshy, dorsoventrally flattened and pointed posteriorly. Mantle extends and forms lateral fins. Head and trunk are joined by narrow neck.
  5. Sepia escapes the observation of enemies by ejecting a black ink-like fluid which forms a smoky screen.
  6. Sexes are separate.
  7. It has ink gland. When irritated or chased by enemies it secretes ink, water becomes coloured and Sepia escapes.
  8. It is used as food.
Sepia (Cuttle Fish)

LOLIGO

Classification of Loligo

PhylumMollusca
ClassCephalopoda
OrderDecapoda
GenusLoligo

Distribution: Cosmopolitan, found along entire Pacific and Atlantic Coast, China and India.

Habits and Habitat:

  1. Marine, carnivorous, feeds on crustaceans, fishes and other molluscs.
  2. It is an active swimmer in open waters of the sea.

Characteristics Feature of Loligo

  1. Commonly known as squid or sea arrow.
  2. Body is fleshy, more or less cylindrical and differentiated into an anterior and posterior trunk.
  3. Head short, contains prominent eyes, 8 arms and two long tentacles and a central mouth. Arms are short, stumpy and non-contractile while 2 tentacles are long, retractile, used for capturing the prey.
  4. In male the arms get filled with spermatophores and is known as hectocotylus which acta as copulatory organ.
  5. Arms and tentacles are provided with suckers.
  6. Eyes are two with olfactory crest.
  7. The mantle extend over the trunk and form two lateral fins.
  8. The shell is internal, siphon is present.
  9. Loligo has got food value.
  10. Ink gland present which emits a cloud of inky fluid helping the animal to escape behind it.
Liligo (A) Dorsal View (B) Ventral View

NAUTILUS

Classification of Nautilus

PhylumMollusca
ClassCephalopoda
OrderTetrabranchiata
GenusNautilus

Distribution: Commonly found in the sea waters in Indo-Pacific region.

Habits and Habitat:

  1. It is an inhabitant of deep-sea, bottom-dweller.
  2. It is a nocturnal and gregarious inhabit.

Characteristics Feature of Nautilus

  1. Commonly known as pearly nautilus.
  2. The body of the animal lies in a tubular and spirally coiled shell, which is internally divided into various chambers by septa.
  3. The chambers increase in size from inner side to outer side of spiral.
  4. Outermost chamber is very large, enclosing the body with head, tentacles and visceral mass.
  5. All the chambers are perforated in the middle through which runs a cord of mantle, called siphuncle.
  6. Except the outermost, all the chambers are filled with gas helping in floating.
  7. The outer surface of the shell bears dark colour bands, while the inner surface is pearly white.
  8. The sexes are separate.
  9. There are 60-90 prehensile tentacles round the mouth.
  10. Nautilus has ornamental and food value.
  11. Ink gland, arms and suckers are absent.
Nautilus (A) Animal with shell (B) Shell in Section

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