Phylum Mollusca- characteristics, classification, examples

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.

Image Source: Wikipedia

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.

References

1. Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
2. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.