Phylum Mollusca Classification, Definition, Characteristics, Examples
Phylum Mollusca Classification, Definition, Characteristics, Examples

Biology

Phylum Mollusca Classification, Definition, Characteristics, Examples

Molluscs, also known as mollusks, are soft-bodied, symmetrical, segmented and coelomate mammals. They are usually shelled with a mantle.

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Definition of Phylum Mollusca

Molluscs, also known as mollusks, are soft-bodied, symmetrical, segmented and coelomate mammals. They are usually shelled with a mantle.

Mollusca animals have soft-bodies. They are triploblastic, bilaterally symmetrical, coelomate and have soft-bodies. Malacology is the study of Mollusca. They are slow invertebrates with thin fleshy mantles or envelopes that cover the visceral organs. Mollusca is a term that Aristotle gave to the cuttlefish. Mollusc means soft.

These organisms can be found both in deep and terrestrial waters. They can range in size from microscopic organisms up to organisms that are 20 meters long.

They are an integral part of the human experience. They can be used as jewellery or as food. These molluscs contain natural pearls.

Bivalve molluscs can be used in marine and freshwater environments as bioindicators. However, few of them, such as snails or slugs, are considered pests.

Examples of Mollusca 

Following are some examples of the animals relating to phylum Mollusca:

  • Pila (Apple snail)
  • Limax (Slug)
  • Unio (Freshwater mussel)
  • Turbinella (Shankha)
  • Helix (Garden snail)
  • Octopus (Devilfish)
  • Loligo (Squid)

Characteristics of Phylum Mollusca

  • They are mostly aquatic, with a few freshwater and some terrestrial forms.
  • They could be hidden parasites within the animal’s interior.
  • They come in a variety of sizes, from large squids or clams to tiny snails that measure a millimeter in length.
  • They are unique in that they have at least two characters of radula or mantle.
  • The body is soft, unsegmented (except for Monoplacophora), bilaterallysymmetrical, coelomate and triploblastic.
  • They are tissue-system rated in body organization
  • The body is composed of the head, foot and visceral masses.
  • The epidermis is a single-layered, often ciliated, covering the body.
  • A calcareous exoskeleton shell is often used to protect the body. It may contain one or more pieces secreted from the mantle.
  • Head is distinct and bears eyes, mouth, and tentacles.
  • The ventral body of the animal is transformed into a muscular, plough-like surface. The foot is then modified for crawling, burrowing, and swimming.
  • The mantle, or pallium, is a fold in a body wall which leaves between itself the main body cavity, the mantle cavity.
  • Visceral mass is the body’s vital organs in a compact form, taking the form dorsal domes or humps.
  • Hemocoel is the body cavity. The coelom can be reduced, and is mainly represented by the pericardial, gonadial, and nephridia.
  • The digestive tract is very simple, with an anterior mouth and posterior andus. However, in cephalopods, scaphodos and gastropods, the intestine changes to a U-shaped shape, which brings anus to the anterior part.
  • Except in pelecypoda, radula, and rasping organs are usually present.
  • Except in cephalopods, the circulatory system is of open type.
  • Many gills and ctenidia are found in the respiratory organs. They are usually paired with osphradiuma at their base. The terrestrial form of the lung is also developed.
  • Direct or indirect respiration, or by the gills, lungs, or both.
  • Their respiratory pigments are Haemocyanin.
  • Excretion can be achieved by paired metanephridia, or kidney.
  • The nervous system is composed of pairs of cerebral, pleural and pedal ganglia, joined by longitudinal and cross-sectional connections and nerves. Ganglia form a circumenteric circle.
  • The sense organs are eyes, statocysts and receptors for touch and smell.
  • Although the sexes are often distinct (dioecious), some are monoecious or hermaphroditic.
  • Fertilization can be external or internal.
  • Development can be done either directly or through metamorphosis at the trochophore stage, veliger larva.

Classification of Phylum Mollusca 

Mollusca (mollusks), are divided into six classes based on their symmetry and the characters of food and shell, mantle and nervous system.

Class 1. Monoplacophora (Gr., monos, one+ plax, plate+ pherein, bearing)

  • The body is bi-symmetrical and segmented.
  • Mantle dome-shaped.
  • The shell is made up of a single piece or a valve.
  • Flattened limpet-shaped shell containing spirally coiled Protoconch.
  • Head without eyes and tentacles.
  • Mantle is a circular fold that encircles the body.
  • Flat and broad foot with 8 pairs pedal retractor muscles.
  • Gills external. 5 pairs of gills are located in pallial grooves.
  • Six pairs of nephridia are available, of which two are gonoducts.
  • Radula in a radular capsule; the intestine is tightly wound.
  • Heart of two pairs of auricles and one ventricle.
  • Nervous system with longitudinal pedal and pedal cords.
  • Sexes separate (dioecious)
  • Examples: Neopilina Galatheae.

Class 2. Amphineura (Gr., amphi, both + neuron, nerve)

  • The body is long with a shorter head.
  • Radula is present
  • Shell in 8 dorsal plates and as spicules
  • Large, muscular, flat, and large ventral foot.
  • The non-ganglionated nerve rings around the mouth are made up of 2 pairs interconnected nerve cords.
  • External fertilization: Trochophore larva.

Subclass 1. Aplacophora

  • Mantle and a worm-like body
  • There is no foot and no shell.
  • Calcareous spicules are buried in the cuticle.
  • Simple radula, marine cavity posterior, and some with a pair bipectinate Ctenidia.
  • Examples: Neomenia, Nematomein, Chaetoderma.

Subclass 2. Polyplacophora

  • Small head; Dorso-ventrally flattened physique
  • No eyes or tentacles.
  • Present: Radula, mantle and foot.
  • Posterior mantle cavity.
  • Shell has 8 calcareous dorsal plate.

Order 1. Lepidopleurina

  • Without insertion plates, the valves of the shell are left open.
  • Ctenidias a few and later.
  • Examples: Lepidopleurus.

Order 2. Chitonida

  • With insertion plates, the valves of the shell are sealed.
  • The entire length of the mantle grooves is covered in gills
  • Examples: Chaetopleura, Chiton, Ischnochiton.

Class 3. Scaphopoda (Gr., Scapha, boat + podos, foot)

  • Only marine
  • The body is bilaterally symmetrical and elongated. It’s enclosed in a tusk shell that opens at both ends.
  • No head; mouth without tentacles; eyes not visible
  • Conical foot, radula present; no gills.
  • The body is completely enclosed by the tubular mantle.
  • The mouth is surrounded by lobular outgrowths or processes.
  • Heart rudimentary.
  • Kidneys paired; gonad single.
  • Sexes separate(dioecious).
  • Trochophore larva.
  • Examples: Dentalium, Cadulus, Pulsellum.

Class 4. Gastropoda (Gr., gaster, belly + podos, foot)

  • There are three types of echinoderms: terrestrial, freshwater, marine, and a few parasitic.
  • Unsegmented body, often with univalve, spirally coil
  • Head distinctive bearing tentacles and eyes.
  • Ventral, broad, flat, and muscular, the foot forms the creeping sole. It often bears dorsally a hard part, the operculumon the posterior end.
  • Torsion (coiling) of body weight at times in development.
  • The mantle, a collar-like piece of body wall that wraps around the body, leaves a gap, the mantle cavities, between the body and itself.
  • The buccal cavity is populated with an odontophore and a radula that contains rows of chitinous tooth.
  • The digestive system includes the muscular pharynx and long esophagus as well as stomach, long-coiled intestine and anteriorly placed anus.
  • Respiration via gills in all forms, through the wall cavity in certain forms and in many other by
  • A pericardium is a protective covering that covers the heart and circulatory system.
  • The excretory system includes metanephridia, which are paired in primitive forms. They can then be reduced to one nephridium when they are used in more complex forms.
  • There are distinct cerebral and pleural components to the nervous system, as well as buccal, pedal and parietal ganglia.
  • In most cases, the sexes are separate (dioecious), while in others they are united.
  • The development includes trochophore as well Veliger

Subclass 1. Prosobranchia(streptoneura)

  • Most of the species are marine and few freshwater, but some terrestrial.
  • Torsionof visceral mass causes the visceral nerve commures to twist into a figure of 8.
  • One pair of tentacles per head
  • The mantle cavity is located anteriorly just in front of the visceral masses.
  • Shell sealed with an operculum, which is carried on foot.
  • Muscular foot forms the ventral part of the body.
  • 2 ctenidia located anterior to the heart in the mantle cavity.
  • Separate sexes (Dioecious); single gonad; larvae trochophore, veliger.

Order 1. Archaeogastropoda (Aspidobranchia)

  • Prosobranchs that lack siphon, penis, proboscis, and penis.
  • One to two bipectinate Ctenidia.
  • The operculum is absent in many forms, with very few exceptions.
  • Shell is usually coiled.
  • 2 kidneys, 2 hearts with 2 auricles
  • 2 osphradia are usually present
  • Nervous system not focused, pedal cord.
  • The right nephridia allows for the direct discharge of sex cells into the sea.
  • Examples: Fissurella (key-hole limpet), Trochus (top shell), Haliotis, Acmaea, Patella, Turbo.

Order 2.  Mesogastropoda (Pectinibranchia)

  • Prosobranchs are usually characterized by siphon, penis, and a non-calcified operaculum.
  • One kidney, one auricle and one mono-pectinate Ctenidium.
  • Radula taenioglossate has 7 teeth per row.
  • Single ospharadium.
  • Without pedal cords, a nervous system.
  • The larva is usually an internal fertiliser.
  • Most of the water is marine. Some freshwater.
  • Examples: Crepidula (slipper shell), Pila (apple snail), Natica (star shell), Hydrobia, Jonthina, Viviparus.

Order 3. Neogastropoda (Stenoglossa)

  • Shell with a siphonal canal that is short or very long.
  • Radula is made up of rows that have 2 or 3 teeth per row.
  • Concentrated nervous system.
  • Osphradium has a large size.
  • Free-swimming veliger suppressed.
  • Examples: Murex, Nassarius, Oliva, Magilus, Buccinum.

Subclass 2. Opisthobranchia

  • Only marine gastropods
  • Shell small, without operculum, or no shell.
  • When present, shell is covered with mantle and pedal cord.
  • Torture or detortion of body mass
  • Gills posterior to your heart.
  • One auricle is located posteriorly to the ventricle in the heart.
  • One kidney, one gonada.
  • Detorsion caused the nervous system to become more concentrated.
  • Monoecious; larva veliger.

Order 1. Cephalaspidea

  • Shell may be present, but it could also be partially or entirely enclosed by the mantle.
  • The tentacular shield protects the head.
  • Lateral parapodial lobes prominent.
  • Examples: Acteon, Hydatina, Bulla.

Order 2. Anaspidea

  • Mostly found in tropical and subtropical water.
  • Mantles usually cover shell less or reduce its thickness.
  • Well-developed parapodial lobes.
  • Anterior ends bear a pair tentacles, two rhinophores, and one pair of eyes.
  • The sperm ducts are open and run the length of the body to the penis located anteriorly.
  • Examples: Aplysia, Akera.

Order 3. Pteropoda

  • Pelagic snails without or with a shell
  • Swimming fins with parapodial fins
  • With or without a mantle cavities
  • Pair of rhinophores for your head
  • Protandrous, open-sperm groove hermaphrodites
  • Examples: Spiratella, Cavolina, Clione, Peraclis.

Order 4. Sacoglossa

  • The shell can be used with or without it.
  • The pharynx has suctorial.
  • Sperm duct closed.
  • Cerata and parapodia are present.
  • Examples: Oxynoe.

Order 5. Acochilidiacea

  • From minute to very small.
  • Without shell or naked snail
  • Parapodia, gills and visceral sacs are visible behind the feet.
  • One can have both the male and female sexes.
  • Inhabit coarse sand.
  • Examples: Acochlidium.

Order 6. Notaspidea

  • Shell internal or reduced shell external.
  • Parapodia absent.
  • Mantle, but not mantle cavity.
  • On the right side, Gills bipectinates and osphradium.
  • Examples: Tylodina, Pleurobranchus.

Order 7. Nudibranchia

  • Shells can be left empty or completely naked.
  • Mantle cavity, internal gills and osphradium are absent.
  • Various dorsal growth.
  • Secondary branchiae are responsible for respiration. They usually form a circle around the anus.
  • Examples: Doris, Eolis, Tritonia, Armina.

Order 8. Pyramidellacea

  • Shell is often spirally twisted.
  • Operculum absent.
  • The absence of radula and Gills is a problem.
  • Long invaginable proboscis.
  • Semi- parasitic.
  • Examples: Turbonilla, Odostomia.

Order . Philinoglossacea

  • Minute naked snail.
  • There are no head appendages or gills.
  • Visceral mass is separated from the foot by a shallow groove.
  • Examples: Philinoglossa.

Order 10. Rhodopacea

  • Vermiform snail.
  • Without external appendages
  • Nephridia Protonephridial type.
  • Anus on the right side.
  • Examples: Rhodope.

Order 11. Onchidiacea

  • Slug-like, without shell opisthobranchs
  • The Mantle Projects extend far beyond the foot.
  • Head is adorned with a pair retractile tentacles, each one tipped by an eye.
  • The posterior end is home to the pulmonary sac, anus, and female gonopores.
  • An anterior location of male gonopore.
  • Examples: Onchidium, Onchidella.

Order 12. Parasita

  • Holothurians have endoparasitic gastropods in their interior.
  • Extremely degenerated snails
  • Shelled embryos
  • Examples: Entoconcha, Thyonicola.

Subclass 3. Pulmonata

  • Most of the species are freshwater, terrestrial or a combination thereof.
  • Shells are usually spiralled or reduced if they are partially or fully covered by the mantle.
  • Np operculum.
  • The mantle cavity is transformed into a pulmonary space with a narrower pore on its right side.
  • Gills absent
  • One auricle is located anteriorly to the ventricles in the heart.
  • Secondarily symmetrical nervous system due to shortening of connectives and concentrations of ganglionic complex.

Order 1. Basommatophora

  • You can choose from freshwater, brackish or marine water.
  • Shell delicate with conical spires and large aperture.
  • One pair of non-invaginable tentacles, with eyes at the bases.
  • The gonopores of male and female are generally distinct.
  • Examples: Siphonaria, Lymnaea, Planorbis.

Order 2. Stylommatophora

  • Terrestrial pulmonates.
  • Shell with conical spire, either internal or absent.
  • Two pairs of retractile or invaginable tentacles, each with eyes at the tips of one of the posterior pairs.
  • The gonopores of male and female are usually merged.
  • Examples: Limax, Helix, Partula, Retinella.

Class 5. Pelecypoda (Gr., pelekus, batchet+ podoa, foot)

  • Some freshwater forms, mostly aquatic, are also available.
  • The body is bilaterally symmetrical, and laterally compressed.
  • Bivalve shells are hinged together and middle-dorsally.
  • Head is not distinct; jaws, pharynxes, radula and tentacles are all part of the same body
  • Ventral is the foot, which is also muscular.
  • Mantle Bilobed, composed of paired, right, and left lobes.
  • Gills and Ctenidia can be paired together, one on either side.
  • The coelom becomes a dorsally located pericardium.
  • Large paired digestive glands are coiled around the alimentary canal.
  • The pericardium contains the heart, which is composed of a median ventricle as well as two auricles.
  • The excretory organ can be paired with nephridia, or kidneys. It opens at one end into the pericardium at its opposite end.
  • The nervous system is composed of four pairs of ganglia, i.e. The nervous system is composed of four pairs of ganglia: cerebral, pleural and pedal.
  • Each side’s cerebral and pleural are usually fused to form a single Cerebropleural ganglion.
  • Statocyst, osphradia and other sense organs are available.
  • The sexes can be either separate or combined.
  • Filter-feeding is the predominant method.
  • Trochophore larvae are often included in the metamorphosis.

Order 1. Protobranchia

  • A single pair of plate-like Ctenidia consisting of two rows of flattened Gills filaments.
  • Placed at the base for muscular proboscides.
  • Stomach style sac
  • The foot is not compressed, but has a flattened ventral area or sole that allows for creeping.
  • Two adductor muscles present.
  • Examples: Nucula, Solenomya.

Order 2. Filibranchia

  • A single pair of plume-like, plume-like gills made of distinct V-shaped filaments.
  • The stomach was equipped with a chitinous gastric shield.
  • Style sac in crystalline style
  • Inter-filamentar junctions can be either absent or created by inter-locking groups of cilia.
  • The inter-lamellar junction can be either absent or nonvascular.
  • There may be two adductor muscles, but the anterior might be smaller or absent.
  • Foot may be small or not well developed.
  • Examples: Mytilus, Arca.

Order 3. Pseudolamellibranchia

  • Plaiting a shaved gill will create vertical folds.
  • Inter-filamentar junctions can be ciliary, vascular or both.
  • Inter-lamellar junctions can be vascular or non-vascular.
  • Single large posterior adductor muscle present.’
  • Shell valves are often equal.
  • Foot rudimentary for feebly developed.
  • Examples: Pecten, Ostraea, Melagrina.

Order 4. Eulamellibranchia

  • Gills are strong and basket-like.
  • The gills filaments became reflexed and fused to form tissue sheets.
  • Gills are used for food gathering.
  • The inter-filamentar junctions and inter-lamellar junctions vascularize the connection between the Gills muscles.
  • Siphons of different sizes are present.
  • Foot can be large or small, but not both.
  • Style short
  • Examples: Anodonta, Unio, Cardium, Venus, Mya, Teredo.

Order 5. Septibranchia

  • There are no gills.
  • Two adductor muscles present.
  • Stomach lined with chitin; style sac decreased.
  • Footlong and slender, and byssus rudimentary and 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 trunk and head.
  • Dorso-ventrally, body elongated.
  • Shells can be external, internal, or absent.
  • Large head with large eyes and a well-developed mouth.
  • The trunk is composed of the symmetrical, uncoiled visceral mass.
  • The mantle surrounds ventrally and posteriorly a large cavity.
  • Foot transformed into a collection of suckers with arms and tentacles around the mouth.
  • Moth bears jaws, radula.
  • 2 to 4 pairs of bipectinate-gills
  • Closed Circulatory system , heart with 2 to 4 auricles
  • There are 2 to 4 pairs of nephridia in the excretory system.
  • The nervous system is very well developed, and the main ganglia are located around the esophagus.
  • Separate the sexes
  • Development meroblastic with metamorphosis.

Subclass 1. Nautiloidea (=Tetrabranchia)

  • Shell external, chambered and spiral.
  • Many suckers in recent species have fewer tentacles.
  • The foot’s main portion surrounds the mouth and is divided into several lobes that bear many tentacles.
  • The funnel is not a complete tube.
  • There are 4 ctenidia, or gills present, as well as 4 kidneys and 4 anuricles.
  • There are no ink glands or chromatophores.
  • Eyes are easy.
  • Examples: Nautilus.

Subclass 2. Smmonoidea

  • Shell external, coiled with complex sutures and septa.
  • Examples: Pachydiscus.

Subclass 3. Coeloidea (=Dibranchia)

  • Shell is usually internally and reduced. It is then enveloped by mantle when it is external.
  • The foot’s main portion is transformed into 8-10 suckers with arms that surround the moth.
  • The funnel is a complete tube.
  • 2 ctenidia, or gills; 2 kidney; 2c auricle; and 2 branchial hearts.
  • There are chromophores and an ink gland duct.
  • Complex structures are found in the eyes.

Order 1. Decapoda

  • Often, the body is elongated with lateral fins.
  • 10 arms – 2 long and called tentacles with suckers at the distal ends, and 8 shorter arms have stalked suckers that are provided with horny edges.
  • Shell is internally developed and well-developed.
  • Usually, there are nidamental glands.
  • Herat is enclosed in the well-developed coelom.
  • Examples: Sepia, Loligo, Spirula.

Order 2. Octopoda

  • The body is globular without fins.
  • 8 arms with sessile suckers, devoid of horny edges.
  • Except for female Argonauta, shell absent
  • Absence of nidamental glands
  • The reduced coelom is not the heart.
  • Examples: Octopus, Agronauta.
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