Phylum Mollusca (Mollusks) Definition

Phylum Mollusca, commonly referred to as mollusks, are segmented, coelomate organisms with soft bodies, bilateral symmetry, and a dorsal visceral mass. They are typically covered in shells.

Phylum Mollusca (Mollusks) Characteristics

The majority of them are aquatic mostly marine, with some terrestrial and rare freshwater forms.

• They could’ve been found within different animals, like hidden parasites.
• These are found in a long range of sizes, from giant squids and clams to microscopic, millimeter long snails.
• At least two of their distinctive personalities are Radula and Mantle.
• The body is coelomate, triploblastic, bilaterally symmetrical, soft, and unsegmented (except in Monoplacophora).
• Their level of bodily organization is tissue-system.
• The body is made up of the head, foot, mantle, and visceral mass.
• The epidermis covering the body is one layer and frequently ciliated.
• The mantle often secretes a calcareous exoskeleton that covers the body in one or more pieces.
• Except in pelecypods and scaphodoa, the head is distinct and carries a mouth, eyes, tentacles, and other sensory organs.
• The foot is differently changed for crawling, burrowing, and swimming, while the ventral body is converted into a muscular surface like a plough.
• A body wall fold known as a mantle or pallium creates a cavity between the main body and the mantle.
• The body’s critical organs are condensed into the visceral mass, which has the appearance of dorsal humps or domes.
• Hemocoel makes up the bodily cavity. The pericardial cavity, gonadial cavity, and nephridia make up most of the decreased coelom.
• The digestive system is straightforward in gastropods, scaphodos, and cephalopods, where the intestine takes on a U-shaped structure and moves the anus to the front half.
• The rasping organs and radula are normally present, with the exception of pelecypoda.
• Except for cephalopods, all organisms possess an open circulatory system.
• Numerous gills or ctenidia, generally with an osphradium at the base, are seen in respiratory organs. Developed in terrestrial animals is the lung.
• The lungs, gills, or both can be used for direct breathing.
• Their respiratory pigments are hemocyanin.
• Paired metanephridia excrete (kidney).
• The paired cerebral, pleural, pedal, as well as visceral ganglia which make up the nervous system are linked to one another via longitudinal and transverse nerves. Typically, ganglia form a ring around the ventricle.
• The sense organs include of eyes, statocysts, and receptors for touch, smell, and taste.
• Certain sexes are monoecious, despite the fact that most are dioecious (separate; hermaphroditic).
• External or internal fertilizations both can occur.
• The veliger larva stage of the trochophore is when development occurs directly or by metamorphosis.

Phylum Mollusca Classification

Depending on their symmetry and the characteristics of their food, shell, mantle, gills, nervous system, muscles, and radula, Mollusca (mollusks) are divided into six groups.

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

• Segmented and bilaterally symmetrical body.
• Dome-shaped mantle.
• The shell is made up of a single valve or component.
• The shell has a protoconch that is spirally wound and flattened.
• Head with tentacles and no eyes.
• The mantle surrounds the body like a ring of folds.
• The foot is wide and flat with eight pairs of pedal retractor muscles.
• Gills outside. Pallial grooves contain five pairs of gills.
• Nephridia come in pairs of six, with two being gonoducts.
• Radula in radular sac; highly coiling intestine.
• a single ventricle and two pairs of auricles make up the heart.
• Pallial and pedal cords run longitudinally in the nervous system.
• Sex differences (dioecious)
• One example is Neopilina galatheae.

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

• Body lengthened and the head shrunk.
• Radula is there.
• Eight dorsal plates, or spicules, make up the shell.
• Big, flat, and muscular ventral foot.
• A non-ganglionated nerve ring with two pairs of linked nerve cords surrounds the mouth.
• Trochophore larval fertilizations occurs externally.

Subclass 1. Aplacophora

• Mantle and worm-like body
• no foot and shell.
• Spicules of calcium are hidden in the cuticle.
• Some have a pair of bipectinate ctenidia in the posterior sea cavity.
• Examples include Neomenia, Nematomein, and Chaetoderma.

Subclass 2. Polyplacophora

• Flattened dorsoventrally, tiny head
• Tentacles but no eyes.
• Present are the radula, mantle, foot, and external gills.
• Cavity behind the mantle.
• The shell is made up of 8 calcareous dorsal plates.

Order 1: Lepidopleurina

• Shell valves are devoid of insertion plates.
• Few and posterior ctenidia.
• Lepidopleurus, as an example.

Order 2: Chitonida

• Shell valves with insertion plates.
• Mantle grooves have gills along the length of them.
• Examples include Ischnochiton, Chaetopleura, and Chiton.

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

• only maritime.
• The body is long, elongated, and covered with a tusk-shell that opens at both ends. It is bilaterally symmetrical.
• No skull, tentacled mouth, and absent eyes.
• No gills, radula present, and conical foot.
• The body is fully encased by a tubular mantle.
• lobular structures or outgrowths surround the mouth.
• Heart is simple.
• pair of kidneys; one gonad.
• Sex differences (dioecious).
• a larval trochophore.
• Examples include Dentalium, Cadulus, and Pulsellum.

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

• Marine, freshwater, terrestrial, and a few echinoderm parasites.
• Unsegmented, generally univalved, and spirally coiled body
• unique head with eyes, mouth, and tentacles.
• The foot is muscular, ventral, broad, flat, and forms the crawling sole. It frequently carries a hard component called the operculum on its dorsal side.
• Body mass can occasionally twist or coil as it develops.
• The mantle is a fold in the body wall that borders the body and mimics a collar; it creates a cavity between itself and the body.
• An odontophore with a radula containing rows of chitinous teeth is found in the buccal cavity.
• The muscular throat, lengthy esophagus, stomach, long coiled intestine, and anteriorly located anus make up the digestive system.
• Most kinds of respiration use gills (ctenidia), others use the wall of the mantle cavity, and many use
• A pericardium surrounds the heart and the open circulatory system.
• The excretory system consists of metanephridia, which frequently only retain one nephridium from pairs in more basic species.
• The nerve system is made up of several buccal, pleural, and cerebral tissues. parietal and visceral ganglia.
• In most forms, the sexes are dioecious, but sometimes they are combined.
• Trochophore and veliger are included in the development.

Subclass 1. Prosobranchia (streptoneura)

• Few freshwater or terrestrial types, primarily marine ones.
• The visceral nerve commissures are twisted into the shape of a “8” as a result of the torsion of the visceral mass.
• One tentacled pair on the head.
• Anteriorly, in front of the visceral bulk, the mantle cavity opens.
• Shell closed by a foot-borne operculum.
• The muscular foot makes up the body’s ventral regions.
• Located anterior to the heart, the mantle cavity contains two ctenidia.
• Dioecious sex separation; solitary gonad; trochophore or veliger larvae.

Order 1. Archaeogastropoda (Aspidobranchia)

• Prosobranchs without a syphon, penis, proboscis, and prostatic glands.
• 1-2 bipectinate ctenidia.
• With rare exceptions, the operculum is also lacking in several forms.
• Usually coils of shell.
• two kidneys, two auricles, and a heart.
• Osphradia 2 are often present.
• Unfocused nervous system with a pedal cable.
• Right nephridia allowed sex cells to exit directly into the water.
• Examples include the keyhole limpet Fissurella, the top shell Trochus, the Haliotis, Acmaea, Patella, and the Turbo.

Order 2. Mesogastropoda (Pectinibranchia)

• Prosobranchs often have a syphon, penis, and an operculum that is not calcified.
• One kidney, one mono-pectinate ctenidium, and one auricle.
• A taenioglossate Radula has seven teeth per row.
• One ospharadium alone.
• a nerve system devoid of foot cords.
• Internal fertilizations occurs; the larva is often a free-swimming veliger.
• Mostly freshwater, but mostly marine.
• Examples include the slipper-shelled Crepidula, apple-snail Pila, star-shelled Natica, Hydrobia, Jonthina, and Viviparus.

Order 3. Neogastropoda (Stenoglossa)

• Shell with a moderately long to extremely long siphonal canal.
• Each row in the Radula has two or three teeth.
• nervous system in focus.
• Osphradium is substantial.
• Suppressed from free-swimming veliger.
• Murex, Nassarius, Oliva, Magilus, and Buccinum are among examples.

Subclass 2. Opisthobranchia

• Only marine-based gastropods.
• Small or without an operculum in the shell.
• When present, mantle or a pedal cord covers the shell.
• Torn or detorted body mass.
• Gills’ back to the heart
• one auricle posterior to the ventricle of the heart.
• One gonad, one kidney.
• Detorsion caused the nervous system to focus.
• veliger larva; monoecious.

Order 1. Cephalaspidea

• Existing shell that the mantle may partially or completely encapsulate.
• Shield your head with your tentacles.
• Large lateral parapodial lobes.
• Acteon, Hydatina, and Bulla, as examples.

Order 2: Anaspidea

• mostly in seas that are tropical or subtropical.
• Mantles often shrink or barely cover the shell.
• Parapodial lobes with good development
• A pair of tentacles, a pair of rhinophores, and a pair of eyes are present on the anterior end.
• Sperm ducts start to open as they travel the length of the body to the anterior penis.
• Example(s): Aplysia, Akera.

Order 3: Pteropoda

• Shelled or shell-less pelagic snails.
• Swimming fins with parapodials.
• Whether it has a mantle cavity or not,
• head with two rhinophores on it.
• Hermaphrodites have a protandrous sperm groove.
• Spiratella, Cavolina, Clione, and Peraclis are among examples.

Order 4. Sacoglossa

• Whether the shell is present or not,
• It is suctorial in the pharynx.
• Sperm duct shut off.
• There are parapodia and cerata.
• Oxynoe is an example.

Order 5. Acochilidiacea

• tiny to miniature.
• Shell less or bare snail.
• Visceral sac, parapodia, and gills protruding behind the foot.
• In some, the sexes are mixed.
• Live in the gritty sand.
• Consider the acochlidium.

Order 6. Notaspidea

• External, reduced, or interior shell.
• Absence of parapodia
• No mantle cavity, only the mantle.
• On the right side, the gills bipectinate and osphradium.
• Tylodina and Pleurobranchus, as examples.

Order 7. Nudibranchia

• Absent or unclothed Shell
• Absence of internal gills, mantle chamber, and osphradium
• Different dorsal growth.
• Secondary branchial respiration often occurs in a circlet around the anus.
• Examples include Doris, Eolis, Tritonia, and Armina.

Order 8. Pyramidellacea

• Usually coiled in a spiral shape.
• Absence of opacity.
• Radula and gills are missing.
• Probing long and invaginable.
• Partially parasitic
• Turbonilla and odostomia are examples.

Order 9: Philinoglossacea

• Little naked snail
• No appendages on the head or gills.
• Just a thin groove separated the visceral mass from the foot.
• For instance, Philinoglossa

Order 10. Rhodopacea

• Vermilion snail
• Without limbs or other extensions.
• Protonephridial kind of nephridia.
• Anus is located on the body’s right side.
• Consider Rhodope.

Order 11. Onchidiacea

• Opisthobranchs with no shells that resemble slugs.
• Mantle extends well beyond the foot.
• The head is supported by two retractable tentacles, each having an eye at the tip.
• At the back are the gonopores for women, the anus, and the pulmonary sac.
• The anterior male gonopore is situated.
• For instance, onchidium and onchidella.

Order 12. Parasita

• Holothurians’ interiors include endoparasitic gastropods.
• snails with severe degeneration.
• Shelled embryos
• Examples: Entoconcha, Thyonicola.

Subclass 3: Pulmonata

• Few marine species, mostly freshwater or terrestrial.
• In most cases, the shell is spiral, diminished, or nonexistent, or if it is there, it is partially or entirely hidden under the mantle.
• Operculum np.
• Right-side pulmonary sac with a small hole emerged from the mantle cavity.
• Absent was Gills.
• One auricle is located prior to the ventricles of the heart.
• Due to the shortening of connectives and concentration of ganglionic complexes, the nervous system is secondarily symmetrical.

Order 1. Basommatophora

• Marine, freshwater, and brackish water organisms.
• Shell delicate with a big aperture and a conical spire.
• One set of eyeballs at the bases of one non-invadable pair of tentacles.
• Normally, male and female gonopore are distinct.
• Siphonaria, Lymnaea, and Planorbis are a few examples.

Order 2. Stylommatophora

• Land-based pulmonates
• Shell with an internal or missing conical spire.
• Two sets of retractable or invasion-capable tentacles, the posterior pair of which have eyes at the tips.
• Normally, the gonopores of the sexes are joined.
• Limax, Helix, Partula, and Retinella, as examples.

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

• Mostly marine, but some freshwater species are aquatic.
• The body is laterally compressed and bilaterally symmetrical.
• Bivalve shells have mid-dorsal hinges that connect them.
• The head, jaws, radula, and tentacles are all vague.
• The foot is muscular in the ventral, or ploughshare, region.
• Bilobed mantle with paired right and left lobes.
• Ctenidia or gills are found in pairs, one on each side.
• The coelom is reduced to a pericardium that is dorsally positioned.
• Large paired digestive glands coil around the alimentary canal.
• The median ventricle and two auricles make up the heart, which is enclosed inside the pericardium.
• The excretory organ is a pair of kidneys, or nephridia, that open to the outside at one end and the pericardium at the other.
• The cerebral, pleural, pedal, and visceral ganglia are the four typical pairs of ganglia that make up the neurological system.
• Each side’s cerebral and pleural tissue often combines to form a single Cerebro-pleural ganglion.
• The statocyst and osphradia are sense organs.
• Sexes can coexist or be divided.
• Mostly filter-feeding.
• A trochophore larva often takes part in the metamorphosis that occurs in conjunction with development.

Order 1. Protobranchia

• Two rows of flattened gill filaments make up each of a single pair of plate-like ctenidia.
• Mouth positioned at the end of the skeletal proboscides.
• Belly with a style sac.
• The sole or ventral surface of the foot is flattened to allow for crawling rather than being crushed.
• There are two adductor muscles.
• Examples are Nucula and Solenomy.

Order 2. Filibranchia

• A single pair of unique V-shaped filaments form gills that resemble plumes.
• The stomach’s chitinous gastric shield is formed.
• Crystalline style type sac.
• Either there are no inter-filamentar connections or they are produced by clusters of interlocking cilia.
• Either the inter-lamellar connection does not exist or it is not vascular.
• There are two adductor muscles; the anterior one may be weak or nonexistent.
• Tiny or underdeveloped feet.
• Mytilus and Arca, as examples.

Order 3. Pseudolamellibranchia

• The plaiting of the gills creates vertical folds.
• Ciliary or vascular inter-filamentar connections are also possible.
• Both vascular and non-vascular inter-lamellar connections exist.
• There is only one substantial posterior adductor muscle.
• Shell valves often have similar sizes.
• Foot is crudely or weakly formed.
• Pecten, Ostraea, and Melagrina, as examples.

Order 4. Eulamellibranchia

• Firm and like a basket.
• To create tissue sheets, gill filaments reflex and entirely fuse.
• Gills serve as food gatherers.
• Vascular inter-filamentar and inter-lamellar connections connect the muscles of the gills.
• There may be a little or massive syphon.
• Large foot, tiny or nonexistent byssus.
• Short sac of style.
• Anodonta, Unio, Cardium, Venus, Mya, and Teredo are a few examples.

Order 5. Septibranchia

• Zero gills
• There are two adductor muscles.
• Chitin lines the stomach; the style sac is smaller.
• Byssus is short, thin, and lacking in rudimentary form.
• Gender equality.
• Poromya and Cuspidaria are two examples.

Class 6. Cephalopoda (=Siphonopoda) (Gr., kephale, head+ podos, foot)

• Both free-swimming and marine.
• On the body, the head and trunk are bilaterally symmetrical.
• Dorso-ventrally stretched out body.
• External, internal, or nonexistent shell.
• Large, prominent head with well-formed eyes and lips.
• The symmetrical, uncoiled visceral bulk that makes up the trunk.
• A sizable mantle chamber is enclosed by the mantle ventrally and posteriorly.
• Foot transformed into an array of suckers with limbs or tentacles surrounding the mouth.
• Moth has a radula and jaws.
• 2 or 4 bipectinate gill pairs.
• closed circulatory system, two to four auricles in the heart
• There are two or four pairs of nephridia in the excretory system.
• The primary ganglia are situated around the oesophagus, and the neural system is well developed.
• Sexes are distinct.
• Metamorphosis-free meroblastic development

Subclass 1. Nautiloidea (=Tetrabranchia)

• External, spiral, and chambered shells.
• There are more suckers and fewer tentacles in recent species.
• The major portion of the foot that surrounds the mouth is separated into lobes with many tentacles.
• A whole tube cannot be formed by the funnel.
• There are 4 auricles, 4 kidneys, and 4 ctenidia or gills.
• There are no chromatophores or ink glands.
• The eyes are basic.
• For instance, Nautilus.

Subclass 2. Smmonoidea

• External, coiled shell with intricate septa and sutures.
• Pachydiscus is one example.

Subclass 3. Coeloidea (=Dibranchia)

• When the shell is external, its cavity is not segmented by septa and is often internal and reduced.
• The foot’s major portion is transformed into 8 or 10 suckers with arms around the moth.
• The funnel creates a whole tube.
• 2 ctenidia or gills, 2 kidneys, 2c auricle, and 2 branchial hearts are present.
• Chromophores and an ink gland duct are seen.
• The architecture of the eyes is intricate.

Order 1. Decapoda

• lateral fins on a typically elongated body.
• Ten arms: two long, referred to as tentacles, with suckers at their distal ends, and eight small arms with stalked suckers and horny rims.
• Internal and well-developed shell.
• Normally, nidamental glands are present.
• Herat is encircled by a sophisticated coelom.
• Examples include Sepia, Loligo, and Spirula.

Order 2. Octopoda

• It has no fins and has a spherical body.
• 8 arms without horny rims and sessile suckers.
• Except for the female Argonauta, shells are lacking.
• Absence of nidimental glands
• The decreased coelom does not contain the heart.
• For instance, octopus and agronauta.

References

• 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.