Phylum Mollusca – Classification, Definition, Characteristics, Examples

What are Phylum Mollusca?

• Phylum Mollusca, ranking as the second-largest phylum in the animal kingdom after Arthropoda, is distinguished by its members known as molluscs or mollusks. This phylum is characterized by approximately 76,000 extant species, with an estimated additional 60,000 to 100,000 fossil species. The high proportion of yet-to-be-described species indicates a vast, unexplored diversity within this group. Molluscs are predominantly marine organisms, accounting for about 23% of all named marine species, although numerous species inhabit freshwater and terrestrial environments. This phylum exhibits remarkable diversity in size, anatomical structure, behavior, and habitat.
• The taxonomic classification of Mollusca is divided into seven or eight classes, including two extinct classes. Notable among these are cephalopods like squids, cuttlefish, and octopuses, recognized for their advanced neurological development. The most numerous class within the phylum is Gastropoda, comprising snails and slugs, which account for 80% of all classified mollusc species.
• Molluscs are characterized by four key features: a predominantly muscular body, a mantle with a significant cavity for breathing and excretion, the presence of a radula (except in bivalves), and a specific nervous system structure. Despite these common attributes, molluscs display immense morphological diversity. This variability often leads to descriptions based on a “hypothetical ancestral mollusc,” featuring a single shell, a muscular foot, and a coelomate yet mainly open circulatory system. The typical mollusc also possesses a complex feeding system, a radula, and a varied reproductive system, ranging from external fertilization to more complex mechanisms.
• The evolutionary history of molluscs traces back to the Cambrian period, with the emergence of gastropods, cephalopods, and bivalves. However, the specifics of their evolution from ancestral Lophotrochozoa and their subsequent diversification remain debated topics.
• Molluscs have significant ecological and economic importance. They serve as a food source for humans but also pose risks due to potential toxin accumulation. Additionally, they have been sources of luxury goods like pearls and dyes, and their shells have historical significance as currency in some societies. Conversely, some mollusc species are considered hazards or pests, with certain species capable of causing serious harm or acting as vectors for diseases like schistosomiasis.
• The term “mollusc” derives from Latin and Greek origins, referring to their soft-bodied nature. Malacology is the scientific study of molluscs. The term “Molluscoida,” once used to group together brachiopods, bryozoans, and tunicates due to perceived similarities to molluscs, has been abandoned due to the lack of significant relations among these groups.
• Molluscs are generally unsegmented, bilaterally symmetrical, and coelomate, often featuring a calcareous shell. Identifying common characteristics across all modern molluscs is challenging due to their varied body structures. However, they typically possess a mantle capable of secreting calcareous structures and a mantle cavity that houses the anus and genitals. The nervous system is typically organized with two pairs of main nerve cords.
• In summary, Phylum Mollusca is a diverse and ecologically significant group in the animal kingdom, characterized by its soft-bodied organisms that exhibit a wide range of anatomical and behavioral adaptations. Their evolutionary history, ecological roles, and economic importance make them a key phylum in biological studies and conservation efforts.

Definition of Mollusca (Mollusks)

Mollusca, commonly known as mollusks, is a large phylum of invertebrate animals characterized by a soft body that is often protected by a hard shell. This diverse group includes species such as snails, slugs, clams, octopuses, and squids. Mollusks are known for their unsegmented bodies, bilateral symmetry, and a mantle that often secretes a calcium carbonate shell. They inhabit a wide range of environments, from marine and freshwater ecosystems to terrestrial habitats. The phylum is distinguished by features such as a muscular foot, used for movement and anchorage, and in many species, a radula, a unique feeding organ.

Characteristics of Phylum Mollusca (Mollusks)

• Habitat and Diversity
  • Mollusca, a diverse phylum, includes both terrestrial (land snails and slugs) and aquatic species (clams, mussels, oysters, octopuses, squids, scallops, etc.). The majority are found in marine and freshwater environments, with a minority existing in terrestrial habitats, particularly in moist soil.
• Organizational Complexity
  • Molluscs exhibit an organ system level of organization, showcasing complex internal structures and functions. This level of complexity allows for specialized systems to perform distinct biological roles.
• Body Symmetry and Germ Layers
  • These organisms display bilateral symmetry and are triploblastic, meaning they develop from three primary germ layers: ectoderm, mesoderm, and endoderm. This layered structure contributes to their complex organ systems.
• Physical Structure
  • The body of molluscs typically includes a head, muscular foot, and a visceral hump. The head features tentacles and compound eyes, serving as primary sensory organs. The muscular foot is crucial for locomotion, and the visceral hump houses essential internal organs.
• Protective Shell
  • Most molluscs possess a calcareous shell, providing protection and structural support. Notable exceptions include octopuses, which lack an external shell.
• Internal Body Cavity
  • Molluscs have a coelom, an internal body cavity, which is crucial for the function of various organ systems.
• Digestive System
  • A well-developed digestive system is present, with the radula functioning as a unique rasping organ for feeding.
• Respiratory Mechanisms
  • Respiration occurs through gills, pulmonary sacs, or the general body surface, depending on the species and its habitat.
• Excretory System
  • The excretion of waste is facilitated by a pair of metanephridia, functioning similarly to kidneys.
• Reproductive Characteristics
  • Molluscs can have separate sexes, while some species are hermaphroditic. Fertilization varies between external and internal, and development is generally indirect, with many species being oviparous.
• Circulatory and Nervous Systems
  • The circulatory system is typically open, with blood flowing freely in the body cavity. The nervous system consists of multiple paired ganglia and nerves, coordinating sensory and motor functions.
• Economic and Ecological Significance
  • Molluscs are not only a key food source but also contribute to jewelry making, notably in the production of natural pearls.

Body Structure of Phylum Mollusca (Mollusks)

The body of mollusks (Phylum Mollusca) is anatomically divided into three primary sections: the head, the muscular foot, and the visceral hump. Each of these components plays a crucial role in the organism’s survival and functionality.

1. Head (Cephalic Region): The head, or cephalic region, is a critical component of mollusks. It typically houses the mouth and sensory tentacles. The mouth is equipped with a radula, a file-like rasping organ unique to mollusks, which is instrumental in their feeding process. The radula’s primary function is to scrape or cut food before ingestion, facilitating efficient nutrient absorption.
2. Muscular Foot: The muscular foot is a distinct feature of mollusks, aiding in locomotion and balance. This structure is embedded with statocysts, which are sensory organs that help in maintaining equilibrium. The foot’s movement capabilities vary among different mollusk species, ranging from simple gliding in snails to more complex jet propulsion in cephalopods.
3. Visceral Hump and Mantle: The visceral hump is located beneath the mantle, which is a soft and spongy layer of skin. The mantle, composed of epithelial cells, serves as a protective covering for mollusks. Additionally, it contains muscle cells that, in some species, facilitate movement through water propulsion. The mantle cavity, situated between the mantle and the visceral hump, houses feather-like gills. These gills are multifunctional, playing a pivotal role in both respiration and excretion. The mantle cavity’s structure and function are critical for the mollusk’s gas exchange and waste removal processes.

Anatomy of Phylum Mollusca (Mollusks)

The anatomy of molluscs (Phylum Mollusca) is characterized by a wide range of diversity. To understand their common features, many textbooks refer to a hypothetical ancestral mollusc (HAM), which shares similarities with modern monoplacophorans.

1. Basic Body Structure
   • The generalized mollusc is an unsegmented, bilaterally symmetrical animal, typically featuring a single, limpet-like shell. This shell is produced by the mantle, which covers the upper surface. The lower part of the mollusc comprises a single muscular foot.
2. Visceral Mass
   • The visceral mass, or visceropallium, represents the soft, nonmuscular metabolic region of the mollusc. It houses the body organs and is integral to the mollusc’s metabolic processes.
3. Mantle and Mantle Cavity
   • The mantle cavity, a significant space within the mantle fold, is lined with epidermis. Its exposure to different environments (sea, freshwater, air) varies among species. The mantle cavity contains the anus, osphradia (chemical sensors), gills, and openings of nephridia (kidneys) and gonads.
4. Shell Structure
   • The mollusc shell, primarily composed of chitin, conchiolin, and calcium carbonate, is secreted by the mantle edge. It generally comprises three layers: an outer organic layer (periostracum), a middle layer of columnar calcite, and an inner nacreous layer.
5. Function of the Foot
   • The ventral muscular foot of molluscs is adapted for various purposes such as locomotion, grasping, burrowing, or feeding. It contains statocysts for balance and, in some molluscs, secretes mucus to aid movement.
6. Circulatory System
   • Most molluscs have an open circulatory system, except cephalopods which have a closed system. The coelom is small, enclosing the heart and gonads, while the main body cavity (hemocoel) circulates blood and coelomic fluid.
7. Respiratory System
   • Molluscs typically possess one or two gills shaped like feathers for respiration. Gills facilitate water circulation in the mantle cavity and are equipped with cilia for filtration and movement of water.
8. Digestive System
   • Molluscs utilize intracellular digestion, with mouths containing radulae for feeding. The radula, unique to molluscs, is a chitinous structure used for scraping food. Food particles are transported to the stomach via cilia-driven mucus strings.
9. Nervous System
   • The molluscan nervous system includes several pairs of ganglia connected by nerve cords. In cephalic molluscs, the nervous system is organized around cerebral, pleural, and pedal ganglia, forming a circumesophageal nerve ring.
10. Reproductive System
    • Molluscan reproduction varies from external to internal fertilization. The simplest form involves trochophore larvae, evolving into more complex veliger larvae or miniature adults. Reproductive organs are connected to the coelom, which plays a role in gamete release and fertilization.

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

Class Monoplacophora (from Greek ‘monos’ meaning one, ‘plax’ meaning plate, and ‘pherein’ meaning to bear) represents a group of mollusks characterized by their unique body structure and shell formation.

• Body Symmetry and Segmentation
  • Monoplacophorans exhibit bilateral symmetry, with a body that is segmented. This segmentation is a notable feature, distinguishing them from many other mollusk classes.
• Mantle and Shell Structure
  • The mantle of Monoplacophora is dome-shaped and encircles the body as a circular fold. This structure supports a single-piece shell, which is typically flattened and limpet-shaped with a spirally coiled protoconch. The shell’s single-valve nature is a defining characteristic of this class.
• Head Features
  • The head of Monoplacophora lacks eyes and tentacles, which differentiates it from several other mollusk classes that possess these sensory organs.
• Foot and Muscle Arrangement
  • The foot of these organisms is broad and flat, equipped with eight pairs of pedal retractor muscles. This muscular arrangement allows for effective movement and interaction with their environment.
• Gill and Respiratory Structure
  • Monoplacophorans have external gills, arranged in five pairs within pallial grooves. These gills play a crucial role in their respiration.
• Excretory System
  • The class has a complex excretory system comprising six pairs of nephridia, two of which also function as gonoducts. This multiple nephridia system is a significant aspect of their internal physiology.
• Digestive System
  • The digestive system features a radula located within a radular sac, and an extensively coiled intestine, adapting them for efficient nutrient absorption and digestion.
• Circulatory System
  • Their circulatory system includes two pairs of auricles and a single ventricle, constituting their heart. This structure ensures effective circulation of blood and nutrients throughout the body.
• Nervous System
  • Monoplacophorans possess a nervous system characterized by longitudinal pallial and pedal cords. This system coordinates their bodily functions and responses to environmental stimuli.
• Reproductive Characteristics
  • The species in this class are dioecious, meaning the sexes are separate. This distinction is vital for their reproductive strategies and lifecycle.
• Representative Species
  • An example of this class is Neopilina galatheae, which exemplifies the distinct characteristics of Monoplacophora.

Class 2. Amphineura

Amphineura, derived from Greek ‘amphi’ (both) and ‘neuron’ (nerve), is a class of mollusks characterized by their distinct nervous system and body structure. This class is divided into two subclasses: Aplacophora and Polyplacophora.

• General Characteristics
  • Members of Amphineura typically have an elongated body with a reduced head. They possess a radula, a feature common in many mollusks. Their shell varies in form, either presenting as eight dorsal plates or as spicules. The ventral foot is large, flat, and muscular, aiding in locomotion. Notably, their nervous system includes a non-ganglionated nerve ring around the mouth and two pairs of interconnected nerve cords. Reproduction occurs externally, producing a trochophore larva.
• Subclass 1: Aplacophora
  • Body Structure: Aplacophora species have a worm-like body covered by a mantle but lack a distinct shell and foot.
  • Skin and Radula: Their skin is embedded with calcareous spicules, and they possess a simple radula.
  • Respiration and Examples: The marine cavity is positioned posteriorly, with some species featuring a pair of bipectinate ctenidia for respiration. Representative examples include Neomenia, Nematomenia, and Chaetoderma.
• Subclass 2: Polyplacophora
  • Body and Head: Polyplacophora species are dorso-ventrally flattened with a small head. They lack eyes and tentacles.
  • Shell and Radula: These mollusks have a radula, mantle, foot, and external gills. The posterior mantle cavity is a significant feature. Their shell consists of eight calcareous dorsal plates.
• Order 1: Lepidopleurina within Polyplacophora
  • Shell Structure: The shell valves lack insertion plates, distinguishing them within the subclass.
  • Gills and Examples: Ctenidia are few and positioned posteriorly. An example of this order is Lepidopleurus.
• Order 2: Chitonida within Polyplacophora
  • Shell and Gills: In contrast to Lepidopleurina, the shell valves in Chitonida have insertion plates. Gills extend along the entire length of the mantle grooves.
  • Representative Species: Notable species include Chaetopleura, Chiton, and Ischnochiton.

Class 3. Scaphopoda

Scaphopoda, derived from Greek ‘Scapha’ (boat) and ‘podos’ (foot), is a class of marine mollusks. This class is recognized for its distinctive tusk-shaped shells and unique anatomical features.

• Habitat
  • Scaphopods are exclusively marine organisms, inhabiting various marine environments.
• Body Structure and Symmetry
  • The body of Scaphopods is bilaterally symmetrical and elongated. It is enclosed within a characteristic tusk-shell, which is open at both ends, resembling a boat in shape.
• Head and Sensory Organs
  • Scaphopods lack a defined head structure. Instead, their mouth is surrounded by tentacles. Notably, they do not possess eyes, which is a unique feature within this class.
• Locomotion and Feeding
  • Their foot is conical in shape, aiding in movement within their marine habitats. Scaphopods possess a radula, a structure common to many mollusks, used for feeding.
• Mantle and Respiratory System
  • The mantle in Scaphopods is tubular, completely encasing the body. Unlike many other mollusks, Scaphopods lack gills, and respiration occurs through the surface of their mantle.
• Digestive and Circulatory Systems
  • The mouth of Scaphopods is surrounded by lobular processes or outgrowths, aiding in feeding. The heart is rudimentary, reflecting a simple circulatory system.
• Excretory and Reproductive Systems
  • Scaphopods have paired kidneys and a single gonad. They exhibit separate sexes (dioecious), with sexual reproduction resulting in the development of a trochophore larva.
• Representative Examples
  • Examples of Scaphopods include species such as Dentalium, Cadulus, and Pulsellum, which exemplify the class’s distinctive characteristics.

Class 4. Gastropoda

Gastropoda, from Greek ‘gaster’ (belly) and ‘podos’ (foot), is a diverse class of mollusks including snails and slugs. They inhabit marine, freshwater, terrestrial environments, and a few are parasitic on echinoderms.

• General Body Characteristics
  • Gastropods have an unsegmented, asymmetrical body, typically with a univalve, spirally coiled shell. The head is distinct, bearing tentacles, eyes, and mouth. The ventral foot is broad, flat, muscular, and functions as the creeping sole, often bearing an operculum.
• Torsion and Mantle
  • Torsion, or coiling of the body mass, occurs at some point in their development. The mantle forms a collar-like fold, lining the body and creating a space, the mantle cavity, between it and the body.
• Feeding Mechanism
  • The buccal cavity contains an odontophore with a radula bearing rows of chitinous teeth, essential for feeding.
• Digestive System
  • The digestive system includes a muscular pharynx, long esophagus, stomach, long coiled intestine, and an anteriorly placed anus.
• Respiratory System
  • Respiration occurs by gills (ctenidia) in most forms, though some respire through the wall of the mantle cavity.
• Circulatory and Excretory Systems
  • Gastropods have an open circulatory system with a heart enclosed in a pericardium. Their excretory system typically comprises metanephridia, paired in primitive forms and reduced to a single nephridium in most.
• Nervous System
  • The nervous system includes cerebral and pleural ganglia, along with buccal, pedal, parietal, and visceral ganglia.
• Reproductive System
  • Most gastropods are dioecious, with some hermaphroditic forms. Their development includes trochophore and veliger stages.
• Subclasses and Orders
  • Gastropoda is divided into several subclasses and orders, each with unique characteristics:
    • Prosobranchia: Mostly marine, with torsion in the visceral mass, and distinct nervous system arrangements.
    • Opisthobranchia: Marine gastropods, often with reduced or absent shells, and various body forms.
    • Pulmonata: Primarily freshwater or terrestrial, with a mantle cavity transformed into a pulmonary sac.
• Examples of Gastropods
  • Representative species include Fissurella (key-hole limpet), Trochus (top shell), Helix (garden snail), and Limax (slug).

Class 5. Pelecypoda

• Overview of Class Pelecypoda
  • Pelecypoda, named from Greek ‘pelekus’ (hatchet) and ‘podos’ (foot), refers to bivalve mollusks. This class includes aquatic species, predominantly marine, with some existing in freshwater environments.
• Body Symmetry and Structure
  • The body of Pelecypods is bilaterally symmetrical and laterally compressed. They are characterized by bivalve shells that are hinged together dorsally. Unlike many mollusks, Pelecypods lack a distinct head, pharynx, jaws, radula, and tentacles.
• Foot and Mantle
  • The foot is ventral, muscular, and ploughshare-shaped, aiding in locomotion. The mantle is bilobed, consisting of paired right and left lobes, covering the internal organs.
• Gills and Coelom
  • Gills, or ctenidia, are paired, one on each side of the body. The coelom is significantly reduced and is primarily represented by a dorsally placed pericardium.
• Digestive System
  • The alimentary canal is coiled with large paired digestive glands, facilitating nutrient processing and absorption.
• Circulatory and Excretory Systems
  • The heart, contained within the pericardium, comprises a median ventricle and two auricles. The excretory system consists of paired nephridia or kidneys, with openings into the pericardium and to the exterior.
• Nervous System
  • The nervous system typically comprises four pairs of ganglia: cerebral, pleural, pedal, and visceral. The cerebral and pleural ganglia are often fused into a single cerebro-pleural ganglion.
• Sense Organs and Reproduction
  • Pelecypods possess statocysts and osphradia as sense organs. They exhibit separate or united sexes and are mostly filter feeders. Development includes metamorphosis with a trochophore larva stage.
• Order 1: Protobranchia
  • Features single pair of plate-like gills and a stomach with a style sac. The foot is not compressed but adapted for creeping. Examples include Nucula and Solenomya.
• Order 2: Filibranchia
  • Possesses single pair of plume-like gills and a chitinous gastric shield in the stomach. Examples include Mytilus and Arca.
• Order 3: Pseudolamellibranchia
  • Characterized by plaited gills forming vertical folds. Examples include Pecten, Ostraea, and Melagrina.
• Order 4: Eulamellibranchia
  • Gills are firm, basket-like, and function in food gathering. Examples include Anodonta, Unio, and Mya.
• Order 5: Septibranchia
  • Distinguished by the absence of gills and the presence of two adductor muscles. Examples include Poromya and Cuspidaria.

Class 6. Cephalopoda

Cephalopoda, from Greek ‘kephale’ (head) and ‘podos’ (foot), comprises marine and free-swimming mollusks. This class is distinguished by its advanced neurological and anatomical features.

• General Body Characteristics
  • Cephalopods exhibit bilateral symmetry, with a body divided into a head and a trunk. The body is elongated dorso-ventrally, and the shell may be external, internal, or absent.
• Head and Mantle
  • The head is distinct and large, equipped with well-developed eyes and a mouth. The mantle encloses a large mantle cavity posteriorly and ventrally.
• Foot Modification and Feeding
  • The foot is modified into a series of sucker-bearing arms or tentacles encircling the mouth. The mouth contains jaws and a radula for feeding.
• Respiratory and Circulatory Systems
  • Cephalopods have 2 or 4 pairs of bipectinate gills. Their circulatory system is closed, with a heart comprising 2 or 4 auricles.
• Excretory System
  • The excretory system includes 2 or 4 pairs of nephridia, responsible for waste removal.
• Nervous System and Reproduction
  • They possess a highly developed nervous system with principal ganglia concentrated around the esophagus. Cephalopods have separate sexes, and their development is meroblastic without metamorphosis.
• Subclass 1: Nautiloidea
  • Nautiloidea are characterized by an external, spiral, chambered shell. They have fewer tentacles with numerous suckers, 4 ctenidia, 4 kidneys, and 4 auricles. Ink glands and chromatophores are absent, and their eyes are simple. Example: Nautilus.
• Subclass 2: Ammonoidea
  • Ammonoidea have an external and coiled shell with complex septa and sutures. Example: Pachydiscus.
• Subclass 3: Coleoidea
  • In Coleoidea, the shell is usually internal and reduced. The foot is modified into 8 or 10 sucker-bearing arms. They have 2 ctenidia, 2 kidneys, 2 auricles, and 2 branchial hearts. Both ink gland duct and chromatophores are present, and their eyes are complex.
• Order 1: Decapoda within Coleoidea
  • Decapods have an elongated body, often with lateral fins, and 10 arms (2 elongated tentacles and 8 short arms). Their internal shell is well-developed, and they usually have nidamental glands. Example: Sepia, Loligo, Spirula.
• Order 2: Octopoda within Coleoidea
  • Octopods have a globular body without fins, and 8 arms with sessile suckers. The shell is generally absent except in female Argonauta, and nidamental glands are absent. Examples: Octopus, Argonauta.

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