Phylum Mollusca: General Characteristics and Its Classification

Mollusca makes the second largest phylum of non-chordate animals including snails, octopuses, sea slugs, squid, and bivalves such as clams, oysters, and mussels.  This phylum contains about 100,000 described species. Among all known marine species, 23% are mollusks. But some species live in freshwater and terrestrial habitats. This group displays a broad range of morphological features such as a muscular foot, a mantle and visceral mass containing internal organs. Calcium carbonate secretes from the mantle and forms outer calciferous body shell in most of the mollusks. The size of the molluscans varies from less than one millimeter to 20 meters. They play an important role in the lives of humans because they are the source of food for many people as well as jewellery. Many molluscans are not good for human’s lives.  Some cause diseases or acts as pests like the snails and slugs. Generally, the hard calciferous shells of mollusks are used to build awesome jewellery pieces. Some mollusks such as bivalves and gastropods produce valuable pearls. Natural pearls are produced when a small foreign object gets trapped in between the mollusk's body shell and mantle. Besides these, many scientists use bivalve mollusks as bioindicators of the freshwater and marine environments.

General Characteristics of Phylum Mollusca

  • They have a bilaterally symmetrical body (exception: gastropods body asymmetrical) and show organ system grade of organization.
  • They are mostly aquatic organisms. Most of them inhabit in the marine environment and few are freshwater. Besides these, some live in terrestrial damp soil.
  • They are triploblastic animals with un-segmented (exception: Neopilina) soft body.
  • Mantle and shell cover the body and the body has three regions namely: head, ventral foot, and a visceral mass. In this case, the shell is a hard calcareous structure, made up of calcium carbonate.
  • They have open type blood circulatory system with heart (one or two auricles and one ventricle) and aorta. The body cavity is known as hemocoel which circulates blood.
  • They have a well-developed and complex digestive system.  The mouth contains a rasping structure ‘radula’ with chitinous teeth for scraping or cutting food.
  • The nervous system consists of three pair of ganglia (cerebral, visceral and pedal) with connectives and nerves.
  • In aquatic mollusks, respiration occurs through gills or ctenidia which are located in the mantle cavity while in terrestrial forms, it occurs through lungs (pulmonary sacs) or general body surface.
  • They have metanephridia (kidney) for excretion.
  • Sense organs consist of eyes, osphradium, tentacles, and statocyst.
  • Sexes are usually separate but some are monoecious. In this case, sexual reproduction occurs.
  • External or internal fertilization occurs.
  • They show direct or indirect development with trochophore or veliger larval stages. Glochidium larva also occurs during the parasitic stage for some species (mussels and clams).
  • They use the ventral muscular foot for locomotion.

Classification of Phylum Mollusca

Phylum Mollusca can be divided into the following different classes:

  • Pelecypoda or Bivalvia
  • Gastropoda
  • Cephalopoda
  • Aplacophora
  • Monoplacophora
  • Polyplacophora
  • Scaphopoda

Class-1: Pelecypoda or Bivalvia

This class comprises more than 15000 species including mussels, oysters, clams, scallops, etc. The largest living bivalve species is giant clam (Tridacna gigas) which can grow up to 47 inches (1,200 mm) in length and more than 200 kg in weight. 

  • They are characterized by a shell which is divided into left and right valves.
  • They are mostly marine organisms and they burrow in mud and sand.
  • They have a bilaterally symmetrical and laterally compressed body.
  • They do not have a distinct head.
  • They are filter feeder but some are scavengers. 

Examples: Mya arenaria (soft-shell clam),  Mercenaria mercenaria (hard clam) 

image of Mya arenaria

Mya arenaria (soft-shell clam)

Class- 2: Gastropoda

The class Gastropoda contains more than 65,000 species which make the largest group in the phylum Mollusca comprising over 80% of all mollusks. Among 65,000 species, about 30,000 are marine, 5,000 inhabit in freshwater, and 30,000 live on land. They are the successful groups in the ocean, freshwater (ponds, streams, marshes, and lakes) and land.

  • They have a well-developed head. Head bears eyes and a pair of tentacles.
  • They have a large and flat muscular foot which is used for creeping, swimming or burrowing.
  • They have a well-developed nervous and circulatory system with the concentration of nerve ganglia.
  • They have a bilaterally symmetrical body with a spiral or coiled shell.

Examples: Turritella cumberlandia, Diastoma insulaemaris, Epitonium charlestonensis, Pila globosa (Apple snail)

image of Pyla globosa

Pila globosa (Apple snail)

Class- 3: Cephalopoda

This class contains about 800 known living species including octopus, squid, and cuttlefish which only inhabit in marine environments. They are carnivorous and their foods consist of various fish, crustaceans and other mollusks.

  • They bear hard beaks for tearing and devouring their prey.
  • They have a  symmetrical body. The left and right side of the body is equal or identical.
  • They have a well-developed vision to detect their pray or potential enemies.
  • They have the ability to squirt ink during threatened.
  • Their foot is modified into eight arms or two tentacles in the head region. In this case, Tentacles are used for capturing prey while arms force prey into the mouth.
  • They have a large pair of vertebrate like eyes. 
  • They are able to detect the polarization of the light.
  • Their brain is large and well-developed in contrast to any other mollusks.
  • They have a very complex nervous system.
  • The shell is either external, internal or absent.

Examples: Octopus abaculus, Loligo vulgaris, Sepia dubia

image of Loligo vulgaris

Loligo vulgaris

Class-4: Monoplacophora

There are about 25 living species which all inhabit the deep sea between 175–6400 m depth ranges.

  • They are marine mollusks characterized by a single, cap-shaped shell.
  • They have a bilaterally symmetrical body with a curved anterior apex, and a ventral foot and mouth.
  • The mouth is located in front of the foot, and the anus is situated in the pallial groove at the posterior end of the body, behind the foot.
  • They do not have a head with no eyes or eyespots.
  • Gills are external that grow in the ventral pallial cavity on either side of the foot.
  • They have a looped digestive system where the stomach is cone-shaped with long intestine that makes four to six loops before reaching the posteriorly-positioned anus.
  • They have multiple pairs of excretory organs (nephridia) which are arranged serially.

Examples: Laevipilina antarctica

 Class-5: Aplacophora

They are a small group of mollusks which contains about 320 species.

  • They are exclusively deep water marine mollusks and are found in all oceans of the world.
  • They have a worm-like appearance with no shells.
  • They are very small sized mollusks and their length ranges from 5 -30 cm. 
  • Mantle secrets tiny calcareous spicules which make their body a beautiful shine.
  • They have no eyes, tentacles, and nephridia but radula is present.

Example: Neomenia yamamotoi

image of Neomenia yamamotoi

Neomenia yamamotoi

Class-6: Polyplacophora (Chitons)

Animals of this class are commonly known as chitons. They are exclusively marine organisms and have a worldwide distribution.

  • They are the familiar group of organisms with eight valves on their shells.
  • Most of the animals of this class inhabit rocky intertidal or shallow subtidal zones but few live in more than 7000 meters depth.
  • They have a broad ventral foot that is used for suction to rocks and other substrates.
  • They have elongate-oval and flattened body. The body contains eight overlapping dorsal plates. 
  • The pallial cavity bears many pairs of small-sized gills which surround the foot.
  • Generally, they are small in size and their length ranges from 0.5-5 cm but one species over 30 cm.
  • The blood circulatory system is open with a heart and for excretion, a pair of kidneys are present that open to the pallial cavity.
  • The nervous system is simple that contains two pairs of lateral nerve cords with several special tiny sensory organs.
  • A simple velum is present which surrounds the mouth and the head does not bear tentacles or eyes.
image of Cryptochiton stelleri

Cryptochiton stelleri (Gumboot or Gumshoe chiton)

Class-7: Scaphopoda

This class contains about more than 350 species. Most species live in fairly deep water within about 4,000 meters depth ranges. Among them, many species are cosmopolitan in distribution. Generally, their foods consist of small organisms such as foraminiferans and young bivalves.

  • They are also known as tusk shells, elephant’s tusk, elephant’s tooth or tooth shells.
  • They live their adult lives buried in sand or mud.
  • They have an elongated and bilateral symmetrical body.
  • They have both ends open single conical shell.
  • They have no eyes but have a radula.
  • The foot is reduced and modified into tentacles. The tentacles have a bulbous end which is known as captaculae.
  • The captaculae are used to grasp and capture prey.
  • They do not have ctenidia and have a pair of kidney.
  • Sexes are separate, and they show the larval stage (trocophore larva) in their life cycle.

Examples: Cadulus delicatulus, Dentalium americanum

image of Dentalium americanum

Dentalium americanum

Concluding Remarks

Mollusca is the second largest phylum which contains over 75,000 described species. Among all known marine species, 23% are mollusks. But some species live in freshwater and terrestrial habitats.They play an important role in the lives of humans because they are the source of food for many people. The hard calciferous shells of mollusks are used to build awesome jewellery pieces. Some mollusks such as bivalves and gastropods produce valuable pearls. Besides these, many scientists use bivalve mollusks as bioindicators of the freshwater and marine environments.

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