Phylum Mollusca: Definition, Characteristics, Classification and Examples

Phylum Mollusca (L., mollis = soft) is a major group in the animal kingdom that includes soft-bodied invertebrates, often characterized by a calcareous shell, though not all members possess one. Molluscs exhibit a wide variety of body forms and habitats, ranging from marine to freshwater and terrestrial environments. Key features of this phylum include a muscular foot used for locomotion, a mantle that secretes the shell (if present), and a visceral mass housing the internal organs. The phylum includes diverse classes such as Gastropoda (snails and slugs), Bivalvia (clams and mussels), Cephalopoda (octopuses and squids), and others. Molluscs play important ecological and economic roles, from maintaining marine ecosystems to serving as food for humans.

* Mollusca is the second largest phylum after Arthropoda. They form one of the most definitely characterized groups of animals.

They have atleast two characters ‘radula and mantle’ not found elsewhere.

General Characteristics of Phylum Mollusca

• Tissue-system grade of body organization.
• They are generally aquatic, mostly marine, few freshwater and some terrestrial forms.
• Triploblastic, coelomate, unsegmented and bilaterally symmetrical.
• Body divisible into head, mantle, foot and visceral mass.
• Body is commonly protected by an exoskeletal calcareous shell of one or more pieces, secreted by the mantle.
  Mantle or pallium is a fold of body wall that leaves between itself and the main body mass, the cavity.
• Head is distinct, bearing the mouth and provided with eyes, tentacles and other sense organs.
• Ventral body is modified into a muscular flat surface, the foot which is variously modified for creeping, burrowing and swimming.
• Visceral mass contains the vital organs of the body in a compact form taking the form of a dorsal lump or dome.
• Coelom reduced and represented mainly by pericardial cavity, gonadial cavity and kidney.
• Digestive gland complete with a digestive gland or liver (hepatopancreas.
• Pharynx contains a rasping organ – the radula.
• Circulatory system is open except in Cephalopods which shows some tendency towards a closed system.
• Respiratory system consists of numerous gills or ctenidia usually provided with osphradium at the base. Lung is developed in terrestrial forms.
• Excretion by a pair of metanephridia (kidneys).
• Nervous system of paired ganglia, connectives and nerves.
• Sense organs include eyes, statocycts and receptors for touch, smell and taste.
• Sexes are usually separate (dioecious) but some are hermaphrodite. Fertilization is external or internal.

Classification of Phylum Mollusca

Note: The classification is adopted from Hyman L.H. (1967) with certain modifications from Parker and Haswell (1965).

CLASS 1. APLACOPHORA OR SOLENOGASTERS

1. Body worm-like, bilaterally symmetrical and cylindrical.
2. Head, mantle, foot, shell and nephridia are absent.
3. Body covered with cuticle beset with numerous calcareous spicules.
4. Mouth and anus are terminal or subterminal at opposite end.
5. Digestive tract straight generally provided with a radula.
6. A pair of coelomoducts in the form of gonoducts opening into the terminal part of intestine or independently.
7. A mid-dorsal longitudinal keel or crest is often present.
8. Sexes united (hermaphroditic) or separate (dioecious).

Order 1. Neomenioidea

• Mid-ventral longitudinal groove present.
• Examples: 1. Neomenia, 2. Proneomenia, 3. Lepidomenia.

Order 2. Chaetodermatoidea

• Mid-ventral longitudinal groove absent.
• Examples: 1. Chaetoderma, 2. Prochaetoderma.

CLASS 2. POLYPLACOPHORA

1. Mostly bilaterally symmetrical, dorsoventrally flattened molluscs.
2. Body elliptical, convex dorsally and flattened ventrally.
3. Head distinct without eyes and tentacles.
4. Shell composed of a longitudinal series of eight calcareous pieces.
5. Foot flat and ventral, laterally bordered by a groove containing gills.
6. Radula well developed comprising 17 teeth.
7. Intestine coiled with a terminal anus.
8. A pair of nephridia extending from pericardium to the lateral groove.
9. A pair of gonoducts without relation to the pericardium.
10. Sexes are separate (dioecious).

Order 1. Lepidopleurida

• Valves of the shell without insertion plates, or if present without insertion teeth.
• Examples: 1. Lepidopleurus, 2. Hanleya.

Order 2. Chitonina

• Valves of the shell with insertion plates and teeth.
• Examples: 1. Tonicella, 2. Chiton, 3. Cryptochiton, 4. Choneplex.

CLASS 3. MONOPLACOPHORA

1. Body bilaterally symmetrical and segmented.
2. Shell comprises single piece or valve.
3. Head without eyes and tentacles.
4. Foot flat and ventral.
5. Mantle encircles the body as a circular fold of the body wall.
6. Gills external and serially arranged.
7. Five pairs of nephridia serially arranged.
8. Sexes separate (dioecious).
9. Example: Neopilina galatheae.

CLASS 4. GASTROPODA

1. Gastropods are marine, freshwater, terrestrial and few parasitic on echinoderms.
2. Body unsegmented, asymmetrical typically with a univalve, spirally coiled shell.
3. Head distinct bearing tentacles, eyes and mouth.
4. Foot is ventral, broad, flat and muscular forming the creeping sole and often bearing dorsally a hard piece, the operculum on its posterior end.
5. Visceral mass spirally coiled exhibiting torsion.
6. Mantle is a collar-like fold of body wall, lining the body whorl leaving a space, the mantle cavity, between itself and the body.
7. Buccal cavity contains an odontophore with a radula bearing rows of chitinous teeth.
8. Digestive system comprises a muscular pharynx, long oesophagus, stomach, long coiled intestine and anteriorly placed anus.
9. Respiration by gills (ctenidia) in most forms, through the wall of the mantle cavity in some forms and in many by lungs.
10. Circulatory system is open and the heart is enclosed in a pericardium.
11. Excretory organs comprise metanephridia which are paired in primitive forms and reduced to a single nephridia in most forms.
12. Nervous system comprises distinct cerebral and pleural beside buccal, pedal, parietal and visceral ganglia.
13. Sexes are separate (dioecious) in most forms, while in some forms united (hermaphroditic).
14. Development includes trochophore and veliger larval stages.

Subclass I. PROSOBRANCHIA

1. Mostly marine, few freshwater or terrestrial forms.
2. Owing to torsion of the visceral mass, the visceral nerve commissures are twisted into a figure of “8”.
3. Mantle cavity opens anteriorly in front of the visceral mass.
4. Shell is generally conical and spirally coiled with an operculum.
5. Head distinct with snout bearing a pair of tentacles and a pair of eyes.
6. Foot is muscular, forms the ventral part of the body.
7. Ctenidia or gills, if present, are situated in front of the heart.
8. Sexes are separate (dioecious).

Order 1. Archaeogastropoda

• Prosobranchs without proboscis, siphon, penis and prostratic glands.
• Operculum is also absent in many forms with few exceptions.
• One or two bipectinate internal gills.
• Heart mostly with two auricles.
• Two osphradia usually present.
• Two nephridia.
• Nervous system not concentrated usually with pedal cords.
• Sex cells discharged directly into the sea by way of the right nephridium.
• Examples: 1. Haliotis, 2. Fissurella, 3. Acmaea, 4. Patella, 5. Trochus, 6. Astraea, 7. Turbo.

Order 2. Mesogastropoda

• Prosobranchs usually with siphon, penis and a non-calcified operculum.
• Radula taenioglossate type having 7 teeth in each row.
• One monopectinate gill.
• Heart with one auricle.
• Single osphradium.
• Single nephridium.
• Nervous system concentrated without pedal cords.
• Examples: 1. Viviparus, 2. Ampullarius, 3. Pila, 4. Valvata, 5. Truncatella, 6. Littorina, 7. Hydrobia, 8. Jonthina, 9. Cypraea.

Order 3. Stenoglossa or Neogastropoda

• Shell with more or less elongated siphonal canal.
• Radula consists of rows with two or three teeth in each row.
• Examples: 1. Murex, 2. Magilus, 3. Buccinum, 4. Melongena, 5. Conus, 6. Terebra.

Subclass II. OPISTHOBRANCHIA

1. Exclusively marine gastropods.
2. Shell often reduced or wanting, when present often covered with mantle or pedal folds.
3. Operculum usually absent.
4. Single gill or often replaced by secondary branchiae in the form of dorsal outgrowths.
5. Heart with one auricle posterior to the ventricle.
6. Due to the detorsion the mantle cavity rotated to the right side or often lost.
7. Nervous system concentrated due to detorsion.
8. Hermaphrodite, i.e., sexes united.

Order 1. Onchidiacea

• Slug-like, naked or without shell opisthobranchs.
• Mantle projects widely beyond the foot.
• Head bears a pair of retractile tentacles each tipped with an eye.
• Pulmonary sac, anus and female gonopore located at the posterior end.
• Male gonopore placed anteriorly.
• Examples: 1. Onchidium, 2. Onchidiella.

Order 2. Cephalaspidea

• Shell is generally present but may be paired or wholly enclosed by mantle.
• Parapodial lobes present or absent.
• Head with tentacular shield.
• Examples: 1. Acteon, 2. Hydatina, 3. Bulla.

Order 3. Anaspidea or Aplysiacea

• Found mostly in tropical and subtropical waters.
• Shell small, more or less covered by mantle.
• Parapodial lobes well developed.
• Anterior end bears a pair of tentacles, a pair of rhinophores and a pair of eyes.
• Sperm duct open, running the body length to the penis located anteriorly.
• Examples: 1. Aplysia, 2. Akera.

Order 4. Pteropoda

• Pelagic snails with or without shell.
• Swim by a pair of lateral expansions.
• Protandrous hermaphrodite, with open sperm groove.
• Examples: 1. Spiratella, 2. Cavolina, 3. Peraclis, 4. Clione.

Order 5. Acochlidiacea

• Minute without shell or naked snail.
• Gills, parapodia and visceral sac projecting behind the foot.
• Sexes united or separate in few.
• Example: Acochlidium.

Order 6. Philinoglossacea

• Minute naked snails.
• Head appendages absent.
• Gills absent.
• Visceral mass separated from the foot only by a shallow groove.
• Example: Philinoglossa.

Order 7. Saccoglossa

• With or without shell.
• Pharynx suctorial.
• Sperm duct is closed.
• Parapodia and cerata present.
• Example: Oxynoe.

Order 8. Notaspidea

• Shell present or absent.
• Parapodia absent.
• Gills bipectinate and osphradium on the right side.
• Mantle present but devoid of mantle cavity.
• Examples: 1. Tylodina, 2. Pleurobranchus.

Order 9. Nudibranchia

• Shell absent or naked.
• Mantle or mantle cavity absent.
• Internal gills or ctenidia and osphradium absent.
• Respiration by secondary branchiae usually arranged in a circlet around the anus.
• Examples: 1. Doris, 2. Tritonia, 3. Armina, 4. Eolis.

Order 10. Rhodopacea

• Vermiform snail.
• Without external appendages.
• Nephridia protonephridial type.
• Example: Rhodope.

Order 11. Pyramidellacea

• Shell spirally twisted.
• Long invaginable proboscis.
• Operculum present.
• Gills and radula absent.
• Examples: 1. Turbonilla, 2. Odostomia.

Order 12. Parasita

• Endoparasitic gastropods found in the interior of holothurians.
• Extremely degenerated snails.
• Examples: 1. Entoconcha, 2. Thyonicola.

Subclass III. PULMONATA

1. Mostly freshwater or terrestrial, a few marine members.
2. Shell typically spiral or reduced or absent, if present partly or completely concealed by mantle.
3. Operculum is absent.
4. Mantle cavity transformed into a pulmonary sac with a narrow pore on the right side. Gill absent.
5. Heart with one auricle anterior to the ventricle.
6. Nervous system secondary symmetrical owing to the shortening of connectives and concentration of ganglia into a circum-oesophageal ganglionic complex.
7. Hermaphroditic.

Order 1. Basommatophora

• Freshwater or brackish water and a marine member.
• Shell delicate with a conical spire and large aperture.
• One pair of non-invaginable tentacles with the eyes at their bases.
• Male and female gonopores generally separate.
• Examples: 1. Siphonaria, 2. Lymnaea, 3. Planorbis.

Order 2. Stylommatophora

• Terrestrial pulmonates.
• Shell with a conical spire, internal or absent.
• Two pairs of invaginable or retractile tentacles with the eyes at the tips of the posterior pair.
• Male and female gonopores usually united.
• Examples: 1. Limax, 2. Helix, 3. Partula, 4. Retinella.

CLASS 5. SCAPHOPODA

1. Exclusively marine.
2. Body bilaterally symmetrical, elongated and enclosed in a tusk-like shell open at both ends.
3. Eyes, tentacles and gills are absent.
4. Mantle tubular completely enclosing the body.
5. Mouth surrounded by lobular processes or outgrowths.
6. Foot is reduced used for digging.
7. Heart rudimentary.
8. Sexes separate.
9. Example: Dentalium.

CLASS 6. PELECYPODA

1. Aquatic, mostly marine, some freshwater forms.
2. Body is bilaterally symmetrical and laterally compressed.
3. Shell consists of two lateral valves, hinged together mid-dorsally.
4. Head is not distinct.
5. Pharynx, jaws, radula and tentacles are absent.
6. Foot is ventral, muscular which is plough share.
7. Mantle is bilobed consisting of paired, right and left lobes.
8. Gills or ctenidia are paired, one on each side.
9. Coelom is reduced to a dorsally placed pericardium.
10. Alimentary canal is coiled with large paired digestive glands.
11. Heart is contained within pericardium and comprises median ventricle and two auricles.
12. Excretory organs are paired nephridia or kidneys open at one end into pericardium at the other end to the exterior.
13. Nervous system consists typically of four pairs of ganglia, viz., cerebral, pleural, pedal and visceral. Cerebral and pleural of each side usually fused into a single cerebro-pleural ganglion.
14. Sense-organs are statocyst and osphradia.
15. Sexes are separate or united.
16. Development is accompanied by metamorphosis which usually includes a trochophore larva.

Order 1. Protobranchiata

• Single pair of plume-like ctenidia, each consisting of two rows of flattened gill filaments.
• Foot is not compressed but has a flattened ventral surface or sole for creeping.
• Two adductor muscles present.
• Examples: 1. Nucula, 2. Solenomya.

Order 2. Filibranchiata

• Single pair of plate-like gills formed of distinct V-shaped filaments.
• Inter-filamentar junctions are either absent or formed by groups of inter-locking cilia.
• Inter-lamellar junctions are either absent or non-vascular.
• Two adductor muscles present, anterior may be reduced or absent.
• Foot small or poorly developed.
• Examples: 1. Mytilus, 2. Arca.

Order 3. Pseudolamellibranchiata

• Gills are plaited so as to form vertical folds.
• Inter-filamentar junctions may be ciliary or vascular.
• Inter-lamellar junctions vascular or non-vascular.
• Single large posterior adductor muscle present.
• Shell valves are frequently unequal.
• Foot rudimentary or feebly developed.
• Examples: 1. Pecten, 2. Ostraea, 3. Melagrina.

Order 4. Eulamellibranchiata

• Gills are firm and basket-like.
• Gill filaments are united by vascular inter-filamentar and inter-lamellar junctions.
• Two equal-sized adductor muscles present.
• Siphon of small or large size present.
• Foot large, byssus small or absent. Examples: 1. Anodonta, 2. Unio, 3. Cardium, 4. Venus, 5. Mya, 6. Teredo.

Order 5. Septibranchiata

• Gills reduced to a horizontal muscular partition dividing the mantle cavity.
• Two adductor muscles present.
• Foot long and slender and byssus rudimentary or absent.
• Sexes united.
• Examples: 1. Poromya, 2. Cuspidaria.

CLASS 7. CEPHALOPODA

1. Exclusively marine.
2. Body bilaterally symmetrical with head and trunk.
3. Shell spiral, chambered or usually with or without reduced shell embedded in the mantle.
4. Head bears large eyes and mouth.
5. Trunk consists of symmetrical and uncoiled visceral mass.
6. Mantle encloses posteriorly and ventrally a large mantle cavity.
7. Foot altered into a series of sucker bearing arms or tentacles encircling the mouth.
8. Mouth bears jaws and radula.
9. Two or four pairs of bipactinate gills.
10. Circulatory system closed, heart with two or four auricles.
11. Excretory system comprises two or four pairs of nephridia.
12. Nervous system is highly developed and the principal ganglia are concentrated around the oesophagus.
13. Sexes are separate.
14. Development meroblastic without metamorphosis.

Subclass I. BELEMNOIDEА OR DIBRANCHIATA

1. Shell usually internal and reduced, enveloped by the mantle, when external its cavity is not divided by septa.
2. Main part of the foot is modified into 8 or 10 sucker bearing arms encircling the mouth.
3. Funnel forms a complete tube.
4. Two ctenidia or gills, two kidneys, two auricles and two branchial hearts present.
5. Ink glands, duct and chromatophores are present.
6. Eyes are complex in structure.

Order 1. Decapoda

• Body is generally enlongated often with lateral line.
• Arms are 10 of which 8 short and 2 long. Two longer arms or tentacles are retractile bearing suckers at their distal ends.
• Eight smaller arms bear stalked suckers provided with horny rims.
• Shell is internal and well developed.
• Nidamental glands are usually present.
• Heart enclosed in the well developed coelom.
• Examples: 1. Sepia, 2. Loligo, 3. Spirula.

Order 2. Octopoda

• Body usually globose and devoid of lateral fins.
• Eight arms with sessile suckers and devoid of horny rims.
• Shell is absent except in female Argonauta.
• Nidamental glands absent.
• Heart does not lie in the reduced coelom.
• Examples: 1. Octopus, 2. Argonauta.

Subclass II. NAUTILOIDEA OR TETRABRANCHIATA

1. Shell is external, spiral and chambered.
2. Main part of the foot encircling the mouth, divided into lobes bearing numerous tentacles.
3. Funnel does not form a complete tube.
4. Four ctenidia or gills, four kidneys and four auricles present.
5. Ink gland and chromatophores absent.
6. Eyes are simple.
7. Example: Nautilus.

Subclass III. AMMONOIDEA

1. Shell of various forms straight to spiral.
2. Siphon external and marginal siphuncle.
3. Usually with frilled septal edges.
4. Extinct forms.
5. Example: Ammonites.