Mollusk – Full Description & 7 Amazing Classes


Last updated on August 31st, 2022 at 12:50 am

The phylum Mollusk are soft-bodied invertebrates from the Mollusca phylum, normally entirely or partially confined in a calcium carbonate shell produced by a soft mantle covering the body. In addition to the vertebrates and bugs, the phylum mollusk are among the most varied groups in the animal kingdom, with almost 100,000 to 150,000 species described.

What Is A Mollusk – Full Description


Mollusk have a muscular foot, which is used majorly for anchorage and mobility and differs in function and shapes, depending upon the kind of mollusk under research study.

In shelled mollusk, this foot is generally the same size as the opening of the shell. The foot is retractable along with an extendable organ. The foot is the ventral-most organ, whereas the mantle is the restricting dorsal organ.

Mollusk is called eucoelomate, however, the coelomic cavity is limited to a cavity around the heart in adult animals. The mantle cavity establishes separately from the coelomic cavity.

The visceral mass exists above the foot, in the visceral bulge. This consists of gastrointestinal, anxious, excretory, reproductive, and breathing systems.

The sp[ecies of mollusk that are specifically aquatic have gills for respiration, whereas some terrestrial types have lungs for respiration. In addition, a tongue-like organ called a radula, which bears chitinous tooth-like decoration, exists in numerous types and serves to shred or scrape food.

The mantle, also called the pallium, is the dorsal skin in mollusks; shelled mollusks are specialized to produce a touch and chitinous calcareous shell.

A lot of mollusks are dioecious animals and fertilization happens externally, although this is not the case in terrestrial mollusk, such as slugs and snails, or in cephalopods.

In some mollusk, the zygote hatches and goes through 2 larval phases, which are trochophore and veliger, prior to ending up becoming a young individual; bivalves might show a 3rd larval phase, glochidia.

Abundance & Distribution


The mollusk has actually adapted to all environments with the exception of air. Essentially marine, gastropods and bivalves consist of freshwater types.

Gastropods have actually likewise adapted to land, with countless types living a totally terrestrial presence. Discovered on rocky, sandy, and muddy substrata, mollusk burrow, crawl, end up being sealed to the surface, or are free-swimming.

Mollusk are seen worldwide, however, there is a prevalence of some groups in particular locations of the world. The close association of lots of molluscan groups with their food source – whether by direct reliance on a particular food supply (e.g., plant-eating, or herbivores) or by participation in the food cycle – limitations their geographical distribution; for instance, bivalves of the Teredinidae family (shipworms) are connected with wood. In basic, cold-water areas support fewer species.

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The Phylum Mollusca

The phylum Mollusca is the primary phylum in marine environments. It is approximated that about 23% of all known marine types are mollusk; there are over 75,000 discussed types, making them the 2nd most diverse phylum of animals.

The name “Mollusca” symbolizes a soft body because the earliest descriptions of mollusk originated from observations of unshelled cuttlefish. Mollusk are mainly a marine group of animals; nevertheless, they are understood to occupy freshwater along with terrestrial environments.

Mollusk show a wide variety of morphologies in each class and subclass, but however share a couple of essential qualities, consisting of a muscular foot, a visceral mass including internal organs, and a mantle that might or might not produce a shell of calcium carbonate.

Phylum Mollusca is a very diverse group of mainly marine types. Mollusk have a significant range of species, varying from big predatory squids and octopus, a few of which shows a high degree of intelligence, to grazing kinds with elaborately shaped and colored shells.

The 7 Classes of Mollusk


  1. Monoplacophora
  2. Aplacophora
  3. Bivalvia
  4. Polyplacophora
  5. Cephalopoda
  6. Gastropoda and
  7. Scaphopoda

The class Aplacophora (that bears no plates) consists of worm-like animals mostly discovered in benthic marine environments. These animals do not have a calcareous shell however have aragonite spicules on their skin. They have a basic mantle cavity and lack eyes, arms, and nephridia, basic excretory organs.

Members of class Monoplacophora (that bears just one plate) possess a single, cap-like shell that confines the body. The morphology of the shell and the underlying animal can differ from circular to ovate. A looped gastrointestinal system, several sets of excretory organs, numerous gills, and a set of gonads exist in these animals.

The monoplacophorans were thought believed to be extinct and were only known by means of fossil records up until the discovery of Neopilina galathaea in 1952. Today, researchers have actually discovered almost 2 dozens of extant species.

Animals in the class Polyplacophora (“that bears numerous plates”) are frequently called “chitons” and bear an armor-like eight-plated shell. These animals have a broad, forward foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the kind of a girdle.

Calcareous spinal columns might exist on the girdle to give security and protection against predators. Respiration is enhanced by ctenidia (gills) that exist ventrally. These animals have a radula that is customized for scraping. The nerve system is basic with just buccal or “cheek” ganglia present at the anterior end. Eyespots are missing in these animals. A single set of nephridia for excretion is present.

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Class Bivalvia (“having 2 shells”) consists of clams, oysters, geoducks, mussels, and scallops. Members of this class are discovered as well as freshwater environments. As the name implies, bivalves are confined in a set of shells (valves are typically called “shells”) that are hinged at the dorsal end by shell ligaments in addition to shell teeth. The total morphology is laterally flattened, and the head area is badly developed.

Statocysts and eyespots might be missing in some types. These animals are suspension feeders – they consume the product, such as plankton, that is suspended in the water around them. Due to their diet plan, this class of mollusk does not have a radula.

Respiration is enhanced by a set of ctenidia, whereas excretion and osmoregulation are through a set of nephridia. Bivalves frequently have a big mantle cavity. In some types, the posterior edges of the mantle might fuse to form 2 siphons that serve to take in and exude water.

Among the functions of the mantle is to produce the shell. Some bivalves like mussels and oysters have the special capability to produce and transfer a calcareous nacre or “mother of pearl” around foreign particles that might go into the mantle cavity. This property has actually been commercially made use of to produce pearls.

Animals in class Gastropoda (“having stomach foot”) consist of widely known mollusk like snails, slugs, conchs, sea hares, and sea butterflies.

Gastropoda consists of shell-bearing types along with types with a minimized shell. These animals are unbalanced and typically provide a coiled shell. Shells might be planospiral (like a garden hose wound up), frequently seen in garden snails, or conispiral, (like a spiral staircase), frequently seen in marine conches.

The visceral mass in the shelled types shows torsion around the perpendicular axis on the center of the foot, which is the crucial attribute of this group, together with a foot that is customized for crawling. Many gastropods bear a head with arms, eyes, and a style.

An intricate radula is used by the gastrointestinal system and helps in the intake of food. Eyes might be missing in some gastropod types. The mantle cavity confines the ctenidia in addition to a set of nephridia.

Class Cephalopoda (“head foot” animals), consists of octopi, squids, cuttlefish, and nautilus. Cephalopods are a class of shell-bearing animals in addition to mollusk with a lowered shell. They show vibrant pigmentation, generally seen in octopi and squids, which are used for camouflage.

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All animals in this class are meat-eating predators and have beak-like jaws at the anterior end. All cephalopods show the existence of an extremely strong nerve system together with eyes, in addition to a closed circulatory system. The foot is lobed and becomes arms, and a funnel, which is used as their mode of mobility.

Suckers exist on the arms of octopi and squid. Ctenidia are confined in a big mantle cavity and are serviced by a big capillary, each with its own heart connected with it; the mantle has siphonophores that help with the exchange of water.

Mobility in cephalopods is enhanced by ejecting a stream of water for propulsion. This is called “jet” propulsion. A set of nephridia exists within the mantle cavity. Sexual dimorphism is seen in this class of animals.

Species members mate and the female then lays the eggs in a safeguarded and remote protected niche. Women of some types take care of the eggs for a prolonged time period and might even end up passing away throughout that time.

Cephalopods such as octopi and squids likewise produce sepia or a dark ink, which is sprayed upon a predator to help in a fast getaway.

Reproduction in cephalopods differs from other mollusks because the egg hatches to produce a juvenile adult without going through the trochophore and veliger larval phases.

In the shell-bearing Nautilus spp., the spiral shell is multi-chambered. These chambers are filled with gas or water to regulate buoyancy. The shell structure in squids and cuttlefish is minimized and exists internally in the kind of a squid pen and cuttlefish bone, respectively.

Members of class Scaphopoda (“having boat feet”) are understood informally as “tusk shells” or “tooth shells,” as apparent when analyzing Dentalium, among a couple of staying scaphopod genera.

Scaphopods are typically buried in the sand with the anterior opening exposed to water. These animals bear a single cone-shaped shell, which has both ends open. The head is basic and extends out of the posterior end of the shell. These animals do not have eyes, however, they have a radula, in addition to a foot customized into arms with a round end, referred to as captaculae. Captaculae serves to control and capture the victim. Ctenidia are missing in these animals.

Reproduction in Mollusk

Mollusk replicates sexually. The majority of types have different male and female sexes. Gametes are launched into the mantle cavity. Fertilization might be external or internal, depending upon the types. Fertilized eggs turn into larvae. There might be several larval phases. Everyone is different from the adult phase. Mollusk has a distinct larval type called a trochophore. It is a small organism with cilia for swimming.

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Ecology of Mollusk

Mollusk resides in the majority of terrestrial, freshwater, and marine environments. The bulk of types live in the ocean. They can be discovered in both deep and shallow water and from tropical to polar latitudes. Mollusk are a significant food source for other organisms, consisting of human beings.

You might have consumed mollusks such as clams, oysters, mussels, or scallops.

Different classes of mollusk have different methods of getting food.

  • Gastropods might be herbivores, predators, or internal parasites. They reside in both terrestrial and water environments. Marine types live primarily in shallow seaside waters. Gastropods use their foot to crawl gradually over rocks, reefs, or soil, trying to find food.
  • Bivalves are usually sessile filter feeders. They reside in both freshwater and marine environments. They use their foot to connect themselves to reefs or rocks or to burrow into mud. Bivalves feed upon plankton and nonliving raw material. They filter the food out of the water as it streams through their mantle cavity.
  • Cephalopods are predators that live in marine environments only. They might be discovered in the open ocean or near the coast. They are either scavengers or predators. They usually consume other invertebrates and fish.


Phylum Mollusca is a big, marine group of invertebrates. Mollusk shows a range of morphological variations within the phylum. This phylum is likewise unique because some members display a calcareous shell as an external method of protection.

Some mollusk has actually evolved a minimized shell. Mollusk are protostomes. The dorsal skin in mollusk is customized to form the mantle, which encloses the mantle cavity and visceral organs.

This cavity is rather unique from the coelomic cavity, which in the adult animal surrounds the heart. Respiration is helped with gills referred to as ctenidia. A chitinous-toothed tongue called the radula exists in many mollusk.

Early growth in some types happens by means of 2 larval phases: trochophore and veliger. Sexual dimorphism is the primary sexual method in this phylum. Mollusk can be divided into 7 classes, each with unique morphological qualities.