Phylum Mollusca

Phylum Mollusca- characteristics, classification, examples

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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.

Phylum Mollusca

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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.

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.

Phylum Mollusca


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