Last Updated on March 23, 2020 by Sagar Aryal
Mollusca (Mollusks) Definition
Molluscs (also know as mollusks) are soft-bodied, bilaterally symmetrical, segmented, coelomate animals; usually shelled having a mantle, ventral foot, anterior head, and a dorsal visceral mass.
Phylum Mollusca (Mollusks) Characteristics
- They are essentially aquatic mostly marine, few freshwater and some terrestrial form.
- They may be found as hidden parasites in the interior of other animals.
- They vary in size from giant squids and clams to little snails, a millimeter long.
- They have at least two characters radula and mantle not found elsewhere.
- The body is soft, unsegmented (except in Monoplacophora), bilaterally symmetrical, coelomate, triploblastic.
- They have tissue-system grade of body organization
- The body consists of head, foot, mantle, and the visceral mass.
- The body is clothed with one-layered often ciliated epidermis.
- The body is commonly protected by an exoskeleton calcareous shell of one or more piece secreted by the mantle.
- Head is distinct, bearing mouth, eyes, tentacles and other sense organs except in pelecypoda and scaphodoa.
- The ventral body is modified into a muscular plough-like surface, the foot which is variously modified for creeping, burrowing and swimming.
- Mantle or pallium is a fold of a body wall that leaves between itself the main body, mantle cavity.
- The visceral mass contains the vital organs of the body in the compact form taking the form of dorsal humps or dome.
- The body cavity is hemocoel. The coelom is reduced and represented mainly by the pericardial cavity, gonadial cavity, and nephridia.
- The digestive tract is simple with anterior mouth and posterior anus but in gastropods, scaphodos, and cephalopods the intestine becomes U-shaped bringing anus to anterior part.
- Rasping organs, radula usually present, except in pelecypoda.
- The circulatory system is open type except in cephalopods.
- Respiratory organs contain numerous gills or ctenidia usually provided with osphradiuma at the base. The lung is developed in terrestrial forms.
- Respiration is direct or by gills or lungs or both.
- Haemocyanin is their respiratory pigments.
- Excretion is by paired metanephridia (kidney).
- The nervous system consists of paired cerebral, pleural, pedal and visceral ganglia joined by longitudinal and transverse connections and nerves. Ganglia usually form a circumenteric ring.
- Sense organs consist of eyes, statocysts, and receptors for touch, smell, and taste.
- Sexes are usually separate (dioecious) but some are monoecious (hermaphroditic).
- Fertilization is external or internal.
- Development is direct or with metamorphosis through the trochophore stage called veliger larva.
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Phylum Mollusca Classification
Mollusca (mollusks) are classified into 6 classes according to their symmetry and the characters of food, shell, mantle, gills, nervous system, muscles, and radula.
Class 1. Monoplacophora (Gr., monos, one+ plax, plate+ pherein, bearing)
- Body is bilaterally symmetrical and segmented.
- Mantle dome-shaped.
- The shell comprises a single piece or valve.
- Flattened limpet-shaped shell with spirally coiled Protoconch.
- Head without eyes and tentacles.
- Mantle encircles the body as a circular fold of the body.
- Foot broad and flat, with 8 pairs of pedal retractor muscles.
- Gills external. 5 pairs of gills in pallial grooves.
- 6 pairs of nephridia, two of which are gonoducts.
- Radula in a radular sac; intestine much coiled.
- Heart of 2 pairs of auricles and a single ventricle.
- Nervous system with longitudinal pallial and pedal cords.
- Sexes separate (dioecious)
- Examples: Neopilina galatheae.
Class 2. Amphineura (Gr., amphi, both + neuron, nerve)
- Body elongated with reduced head.
- Radula present.
- Shell as 8 dorsal plates or as spicules.
- Ventral foot, large, flat and muscular.
- Non- ganglionated nerve ring around the mouth with 2 pairs of the interconnected nerve cord.
- External fertilization: trochophore larva.
Subclass 1. Aplacophora
- Worm-like body with mantle
- No shell and foot.
- Calcareous spicules buried in the cuticle.
- Simple radula; marine cavity posterior, some with a pair of bipectinate ctenidia.
- Examples: Neomenia, Nematomein, Chaetoderma.
Subclass 2. Polyplacophora
- Dorso-ventrally flattened body; small head
- No eyes and tentacles.
- Radula, mantle, foot and external gills present.
- Posterior mantle cavity.
- Shell as 8 calcareous dorsal plates.
Order 1. Lepidopleurina
- Valves of the shell without insertion plates.
- Ctenidia a few and posterior.
- Examples: Lepidopleurus.
Order 2. Chitonida
- Valves of the shell with insertion plates.
- Gills along the whole length of mantle grooves
- Examples: Chaetopleura, Chiton, Ischnochiton.
Class 3. Scaphopoda (Gr., Scapha, boat + podos, foot)
- Exclusively marine.
- The body is bilaterally symmetrical, elongated and enclosed in a tusk-shell opens at both ends.
- No head; mouth with tentacles; no eyes.
- Conical foot, radula present; no gills.
- Mantle tubular completely enclosing the body.
- Mouth surrounded by lobular processes or outgrowths.
- Heart rudimentary.
- Kidneys paired; gonad single.
- Sexes separate(dioecious).
- Trochophore larva.
- Examples: Dentalium, Cadulus, Pulsellum.
Class 4. Gastropoda (Gr., gaster, belly + podos, foot)
- Marine, freshwater, terrestrial and few parasitic on echinoderms.
- Body unsegmented, asymmetrical typically with univalve, spirally coiled
- Head distinct bearing tentacles, eyes, and mouth.
- The foot is ventral, broad, flat and muscular forming the creeping sole and often bearing dorsally a hard piece, the operculum on its posterior end.
- Torsion (coiling) of body mass at sometimes in development.
- The mantle is a collar-like fold of body wall lining the body leaving a space, the mantle cavity, between itself and the body.
- The buccal cavity contains an odontophore with a radula bearing rows of chitinous teeth.
- The digestive system comprises muscular pharynx, long esophagus, stomach, long coiled intestine, and anteriorly placed anus.
- Respiration by gills (ctenidia) in most forms, through the wall of the mantle cavity in some forms and in many by
- The open circulatory system and heart is enclosed in a pericardium.
- The excretory system comprises metanephridia which are paired in primitive forms and reduced to a single nephridium in most forms.
- The nervous system comprises distinct cerebral and pleural besides buccal, pedal, parietal and visceral ganglia.
- Sexes separate (dioecious) in most forms while in some forms united.
- The development includes trochophore and veliger
Subclass 1. Prosobranchia(streptoneura)
- Mostly marine, few freshwater or terrestrial forms.
- Owing to torsion of the visceral mass, the visceral nerve commissures are twisted into a figure of “8”.
- Head with a single pair of tentacles.
- Mantle cavity opens anteriorly in front of the visceral mass.
- Shell closed by an operculum borne on foot.
- The foot is muscular, forms the ventral parts of the body.
- 2 ctenidia in mantle cavity situated anterior to the heart.
- Sexes separate (Dioecious); gonad single; larvae trochophore or veliger.
Order 1. Archaeogastropoda (Aspidobranchia)
- Prosobranchs without proboscis, siphon, penis and prostatic glands.
- One or two bipectinate ctenidia.
- The operculum is also absent in many forms with few exceptions.
- Shell usually coiled.
- 2 kidneys and heart with 2 auricles.
- 2 osphradia usually present.
- Nervous system not concentrated, with pedal cord.
- Sex cells discharged directly into the sea by way of the right nephridia.
- Examples: Fissurella (key-hole limpet), Trochus (top shell), Haliotis, Acmaea, Patella, Turbo.
Order 2. Mesogastropoda (Pectinibranchia)
- Prosobranchs usually with siphon, penis and a non-calcified operculum.
- One auricle, one kidney, and one mono-pectinate ctenidium.
- Radula taenioglossate type having 7 teeth in each row.
- Single ospharadium.
- A nervous system without pedal cords.
- Fertilization is internal; larva usually a free-swimming veliger.
- Mostly marine, some freshwater.
- Examples: Crepidula (slipper shell), Pila (apple snail), Natica (star shell), Hydrobia, Jonthina, Viviparus.
Order 3. Neogastropoda (Stenoglossa)
- Shell with a short to a very long siphonal canal.
- Radula consists of rows with 2 or 3 teeth in each row.
- Nervous system concentrated.
- Osphradium is large.
- Free-swimming veliger suppressed.
- Examples: Murex, Nassarius, Oliva, Magilus, Buccinum.
Subclass 2. Opisthobranchia
- Exclusively marine gastropods.
- Shell small without operculum or no shell.
- Shell when present covered with mantle or pedal cord.
- Body mass torted or detorted.
- Gills posterior to the heart.
- Heart with one auricle posterior to the ventricle.
- One kidney, one gonad.
- The nervous system concentrated due to detorsion.
- Monoecious; larva veliger.
Order 1. Cephalaspidea
- Shell present but may be partly or wholly enclosed by the mantle.
- Head with the tentacular shield.
- Lateral parapodial lobes prominent.
- Examples: Acteon, Hydatina, Bulla.
Order 2. Anaspidea
- Found mostly in tropical or subtropical waters.
- Shell usually reduced or less covered by mantles.
- Well-developed parapodial lobes.
- Anterior end bears a pair of tentacles, a pair of rhinophores and a pair of eyes.
- Sperm ducts open, running the body length to the penis located anteriorly.
- Examples: Aplysia, Akera.
Order 3. Pteropoda
- Pelagic snails with or without a shell.
- Parapodial fins for swimming.
- With or without a mantle cavity.
- Head with a pair of rhinophores.
- Protandrous, hermaphrodites with an open sperm groove.
- Examples: Spiratella, Cavolina, Clione, Peraclis.
Order 4. Sacoglossa
- With or without the shell.
- The pharynx is suctorial.
- Sperm duct closed.
- Parapodia and cerata present.
- Examples: Oxynoe.
Order 5. Acochilidiacea
- Minute to small-sized.
- Without shell or naked snail.
- Gills, parapodia and visceral sac projecting behind the foot.
- Sexes united or separate in a few.
- Inhabit coarse sand.
- Examples: Acochlidium.
Order 6. Notaspidea
- Shell external or reduced and internal.
- Parapodia absent.
- Mantle, but no mantle cavity.
- Gills bipectinate and osphradium on the right side.
- Examples: Tylodina, Pleurobranchus.
Order 7. Nudibranchia
- Shell absent or naked.
- Internal gills, mantle cavity and osphradium absent.
- Various dorsal growth.
- Respiration by secondary branchiae usually arranged in a circlet around the anus.
- Examples: Doris, Eolis, Tritonia, Armina.
Order 8. Pyramidellacea
- Shell typically spirally twisted.
- Operculum absent.
- Gills and radula are absent.
- Long invaginable proboscis.
- Semi- parasitic.
- Examples: Turbonilla, Odostomia.
Order 9. Philinoglossacea
- Minute naked snail.
- No gills and head appendages.
- Visceral mass separated from the foot only by a shallow groove.
- Examples: Philinoglossa.
Order 1o. Rhodopacea
- Vermiform snail.
- Without external appendages.
- Nephridia Protonephridial type.
- Anus on the right side of the body.
- Examples: Rhodope.
Order 11. Onchidiacea
- Slug-like, naked without shell opisthobranchs.
- Mantle projects widely beyond foot.
- Head bears a pair of retractile tentacles each tipped with an eye.
- Pulmonary sac, anus and female gonopores located at the posterior end.
- Male gonopore located anteriorly.
- Examples: Onchidium, Onchidella.
Order 12. Parasita
- Endoparasitic gastropods found in the interior of holothurians.
- Extremely degenerated snails.
- Shelled embryos.
- Examples: Entoconcha, Thyonicola.
Subclass 3. Pulmonata
- Mostly freshwater or terrestrial, a few marine forms.
- Shell typically spiral or reduced or absent, if present partly or completely concealed by the mantle.
- Np operculum.
- Mantle cavity transformed into a pulmonary sac with a narrow pore on the right side.
- Gills absent.
- Heart with one auricle anterior to the ventricles.
- Nervous system secondarily symmetrical owing to the shortening or connectives and concentration of ganglionic complex.
Order 1. Basommatophora
- Freshwater, brackish water and marine forms.
- Shell delicate with a conical spire and large aperture.
- One pair of non-invaginable tentacles with eyes at their bases.
- Male and female gonopore generally separate.
- Examples: Siphonaria, Lymnaea, Planorbis.
Order 2. Stylommatophora
- Terrestrial pulmonates.
- Shell with conical spire, internal or absent.
- 2 pairs of invaginable or retractile tentacles with eyes at tips of the posterior pair.
- Male and female gonopore usually united.
- Examples: Limax, Helix, Partula, Retinella.
Class 5. Pelecypoda (Gr., pelekus, batchet+ podoa, foot)
- Aquatic, mostly marine, some freshwater forms.
- The body is bilaterally symmetrical and laterally compressed.
- Bivalve shells hinged together and mid-dorsally.
- Head is not distinct; pharynx, jaws, radula, and tentacles
- The foot is ventral, muscular which is ploughshare.
- Mantle bilobed, consisting of paired, right and left lobes.
- Gills or ctenidia are paired, one on each side.
- The coelom is reduced to a dorsally placed pericardium.
- The alimentary canal is coiled with large paired digestive glands.
- The heart is contained within the pericardium and comprises a median ventricle and two auricles.
- The excretory organ is paired nephridia or kidneys opens at one end into pericardium at the other end to the exterior.
- The nervous system consists typically of 4 pairs of ganglia i.e. cerebral, pleural, pedal and visceral.
- Cerebral and pleural of each side usually fused into a single Cerebro-pleural ganglion.
- Sense organs are statocyst and osphradia.
- Sexes are separate or united.
- Mostly filter-feeding.
- Development is accompanied by metamorphosis which usually includes a trochophore larva.
Order 1. Protobranchia
- Single pair of plate-like ctenidia each consisting of 2 rows of flattened gills filaments.
- Mouth placed at the base of muscular proboscides.
- Stomach with style sac.
- The foot is not compressed but has a flattened ventral surface or sole for creeping.
- Two adductor muscles present.
- Examples: Nucula, Solenomya.
Order 2. Filibranchia
- Single pair of plume-like gills formed of distinct V-shaped filaments.
- Chitinous gastric shield in the stomach developed.
- Style sac with crystalline style.
- Inter-filamentar junctions are either absent or formed by groups of inter-locking cilia.
- The inter-lamellar junction is either absent or non-vascular.
- Two adductor muscles present, anterior may be reduced or absent.
- Foot small or poorly developed.
- Examples: Mytilus, Arca.
Order 3. Pseudolamellibranchia
- Gills are plaited so as to form vertical folds.
- Inter-filamentar junctions may be ciliary or vascular.
- Inter-lamellar junctions are vascular and non-vascular.
- Single large posterior adductor muscle present.’
- Shell valve are frequently equal.
- Foot rudimentary of feebly developed.
- Examples: Pecten, Ostraea, Melagrina.
Order 4. Eulamellibranchia
- Gills are firm and basket-like.
- Gills filaments reflexed and fused completely to form tissue sheets.
- Gills function for food gathering.
- Gills muscles are united by vascular inter-filamentar and inter-lamellar junctions.
- Siphon of small or large size present.
- Foot large, byssus small or absent.
- Style sac short.
- Examples: Anodonta, Unio, Cardium, Venus, Mya, Teredo.
Order 5. Septibranchia
- No gills.
- Two adductor muscles present.
- Stomach lined by chitin; style sac reduced.
- Footlong and slender and byssus rudimentary or absent.
- Sexes united.
- Examples: Poromya, Cuspidaria.
Class 6. Cephalopoda (=Siphonopoda) (Gr., kephale, head+ podos, foot)
- Marine and free-swimming.
- The body is bilaterally symmetrical with head and trunk.
- Body elongated dorso-ventrally.
- Shell external, internal or absent.
- Head distinct and large with well-developed eyes and mouth.
- The trunk consists of the symmetrical and uncoiled visceral mass.
- Mantle encloses posteriorly and ventrally a large mantle cavity.
- Foot altered into a series of suckers bearing arms or tentacles encircling the mouth.
- Moth bears jaws and radula.
- 2 or 4 pairs of bipectinate gills.
- Circulatory system closed, heart with 2 or 4 auricles.
- The excretory system comprises 2 or 4 pairs of nephridia.
- The nervous system is highly developed and the principal ganglia are concentrated around the esophagus.
- Sexes separate.
- Development meroblastic without metamorphosis.
Subclass 1. Nautiloidea (=Tetrabranchia)
- Shell external, spiral and chambered.
- Recent species with many suckers fewer tentacles.
- The main part of the foot encircling the mouth, divided into lobes bearing numerous tentacles.
- The funnel does not form a complete tube.
- 4 ctenidia or gills, 4 kidneys and 4 auricles present.
- Ink glands and chromatophores are absent.
- Eyes are simple.
- Examples: Nautilus.
Subclass 2. Smmonoidea
- Shell external and coiled with complex septa and sutures.
- Examples: Pachydiscus.
Subclass 3. Coeloidea (=Dibranchia)
- Shell usually internal and reduced, enveloped by mantle, when external its cavity is not divided by septa.
- The main part of the foot is modified into 8 or 10 suckers bearing arms encircling moth.
- The funnel forms a complete tube.
- 2 ctenidia or gills, 2 kidney, 2c auricle, and 2 branchial heart presents.
- Ink gland duct and chromophores present.
- Eyes are complex in structures.
Order 1. Decapoda
- Body elongated often with lateral fins.
- 10 arms- 2 elongated and called tentacles bearing suckers at their distal ends and 8 short arms bear stalked suckers provided with horny rims.
- Shell is internal and well-developed.
- Nidamental glands are usually present.
- Herat enclosed in the well-developed coelom.
- Examples: Sepia, Loligo, Spirula.
Order 2. Octopoda
- Body globular and without fins.
- 8 arms with sessile suckers and devoid of horny rims.
- Shell absent except in female Argonauta.
- Nidamental glands absent.
- The heart does not lie in the reduced coelom.
- Examples: Octopus, Agronauta.
- Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
- Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.