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Phylum Mollusca, Practice Problems and FAQs

People like pearl jewelleries. They are very beautiful to see and they really give a classic look to any attire you wear. But why are those jewellery very costly?. It is because pearls are costly. Do you know where the pearls come from?

Yes, you are right, pearls are obtained from an animal called a pearl oyster. Have you ever seen this animal who bears pearls? This question might seem irrational but nature never fails to surprise us when it comes to being unique and creative. 



                     Fig: Pinctada (Pearl oyster)

The time it takes to make a pearl might vary from 6 months to many years. In order to make a pearl, the pearl oyster must reach maturity. It takes around 3 years. When a Mollusc reaches this mature age, it can begin the pearl-making process either naturally or with the assistance of humans. Pearls are normally made by freshwater mussels and marine oysters as a natural defence against an irritant. It can be a parasite entering their shell or something which damages their fragile body. The mussel or oyster slowly secretes layers of conchiolin and aragonite materials that also make up its shell.

Oysters that can only produce pearls once are slaughtered and sold for their meat and other parts, rather than being released back into the oceans. When the pearl is cut out, the ones who could be nucleated again may also die by accident. This pearl oyster belongs to phylum Mollusca, in which most of the animals have the shells around them. So now let’s take a deep dive into the details of the phylum Mollusca.

Table of contents

Phylum Mollusca 

The term ‘Mollusca’ is derived from the term ‘mollis’ which means ‘soft’. This phylum includes the soft-bodied animals. It is the second largest phylum under the animal kingdom. Organisms of phylum Mollusca are characterised by presence of hard calcareous shells. Now we will discuss some of the general characteristics of the phylum Mollusca. 

General characteristics of phylum Mollusca

Habitat

Molluscs are well-adapted to terrestrial as well as aquatic (both freshwater and marine) environments.

Exoskeleton

They have a hard outer calcareous shell, which is secreted by mantle. It may be external or internal. There are few Molluscs without shells too.



                                   Fig: Calcareous shell in snail

Symmetry

Molluscs are bilaterally symmetrical (the body can be divided into two equal and identical halves, i.e., right half and left half, through only one plane). Some, on the other hand, are asymmetrical as a result of torsion or twisting during growth.



            Fig: Bilateral symmetry in Molluscs

Level of organisation

Molluscs have an organ system level of body organisation (where division of labour can be seen between different organ systems). 

Germ layer organisation

Organisms belonging to the kingdom Mollusca are triploblastic. That means they have three germ layers which include the outer ectoderm, middle mesoderm and inner endoderm.



                       Fig: Triploblastic organisation in molluscs

Body cavity

They have a body cavity between the body wall and gut wall which is lined by mesoderm, hence are coelomates. The pericardial, excretory, and reproductive cavities make up the coelom, which is substantially reduced. Blood is stored in the gaps between viscera and forms haemocoel.



                                  Fig: Coelom in molluscs

Segmentation

The body of molluscs is unsegmented. 

Body plan of molluscs

The body of molluscs is divided into three distinct parts as head, muscular foot and visceral hump.



                         Fig: Body parts of a mollusc

Head 

The head is anterior. It possesses sense organs like tentacles and radula. 

Tentacles

Tentacles are present on the forehead of molluscs and help in sensory perception and feeding. 



                       Fig: Tentacles in snail

Radula

It is present in the mouth and acts as a rasping organ. It is used for feeding only and can be compared with a tongue. It is a chitinous ribbon which is minutely toothed. It is typically used for cutting or scraping of food before it enters the oesophagus.



                            GIF: Radula in snail

Muscular foot

It is a soft and muscular organ which is present on the ventral side. It aids in locomotion and anchorage.

Visceral hump

The visceral hump is covered by a spongy and soft layer of skin known as the mantle or pallidium. A cavity is formed between the hump and mantle which is known as the mantle cavity or pallial cavity. Mantle cavity consists of gills which help in respiration and excretion. 



                                       Fig: Gaseous exchange in molluscs

Organ systems of molluscs

The following are the major systems of molluscs. 

Digestive system

The digestive tract is complete, That means it possesses mouth and anus. The radula, a rasping organ with a file-like transverse row of chitinous teeth, is found in the mouth. Anus is opened into the mantle cavity. 

Respiratory system

Gills or ctenidia in the mantle cavity are used for breathing in aquatic forms. The ctenidia cilia normally carry water over the gill filaments in a direction which is opposite to that of the flow of the hemolymph in the underlying ctenidia vessels. This countercurrent phenomenon of water and hemolymph movement makes sure that the gas exchange occurs between the hemolymph and water. This maximises the diffusion gradients of O2 and CO2. Lungs or air sacs are used to breathe in terrestrial animals. 

Circulatory system 

It has an open heart with a pulsatile dorsal heart and a few arteries that open into sinuses. Haemocyanin, a copper-containing blue respiratory pigment, is found in blood plasma.

Excretory system

One or two pairs of excretory glands are present in the molluscs. These serve as excretory organs. They are metanephridia performing the function of kidneys. They are also called organs of Bojanus. They open into the mantle cavity.

Nervous system

It is made up of a few pairs of ganglia that are connected by nerve connections and commissures.

Sense organs

They include eyes, tentacles, statocysts for balance, and osphradia. 

Osphradia

It is used for assessing the chemical and physical properties of water.

Mode of reproduction and development of molluscs

They show sexual reproduction. 

Sexual nature

Molluscs are dioecious in nature, i.e., they have separate sexes.

Mode of birth

Molluscs are oviparous (females lay fertilised or unfertilised eggs inside which the development of embryo occurs).



                                  Fig: Eggs of molluscs

Type of development

Development of molluscs are direct or indirect. In indirect development larval stages are present which then transform into adults.

Common larval stages 

Trochophore larva - It is present in Chiton

Veliger larva - Present in marine snails.

Glochidium larva - It is present in Pinctada

Fig: Indirect development in molluscs

Classification of phylum Mollusca

The animals of this phylum are classified into six classes. They are as follows:

  • Monoplacophora
  • Amphineura
  • Scaphopoda
  • Gastropoda
  • Pelecypoda
  • Cephalopoda

Class Monoplacophora

The shell here is either dome-shaped or spoon-shaped. They have features of both Annelida and Mollusca. Internal organs show metameric segmentation like Annelida. Examples include Neopilina. It is considered as a connecting link between Mollusca and Annelida. It is a living fossil.



                                           Fig: Neopilina

Class Amphineura

They have an elongated body. Shell is either present or absent. Their heads are smaller, with no eyes or tentacles. They also have a nerve ring around the mouth. Examples include Chaetopleura (Chiton).

Chaetopleura 

Chiton is a common name for it. It is a sedentary and marine mollusc. It is nocturnal and feeds on plants. When disturbed, it coils ventrally. It features a ventral foot-like sole and a shell made up of eight plates in a row.



        Fig: Chaetopleura (Chiton)

Class Scaphopoda

The shell is tubular in structure and is open at both the ends. They also have a rudimentary head. The foot is conical and is used for digging purposes. Examples include Dentalium (Tusk shell).

Dentalium 

It can be found in shallow to deep waters. It has a worm-like body with a tubular shell around it. It possesses a conical foot for digging.



                         Fig: Dentalium (Tusk shell)

Class Gastropoda

They are the largest class of phylum Mollusca. A single shell is present which is spirally coiled. Due to torsion in the shell, they are asymmetrical. It is the largest class. They have a distinct head. Eyes and tentacles are present on the head as sensory organs. The foot is muscular and flat. They show indirect development with the veliger larval stage. Bilateral symmetry exists in the larval stage, however this symmetry is lost in adults due to torsion or twisting. Examples include Pila (Apple snail), Aplysia (Sea hare), Cypraea (Old currency) and Limax (the slug).

Pila

The apple snail is its popular name. It is a mollusc that lives in ponds, rivers, and rice fields. It eats plants and breathes through gills in the water and lungs on land. It has a spirally coiled shell with a large opening at the bottom where the head and foot emerge.



                              Fig: Pila (Apple Snail)

Limax

It is also known as the grey slug. It prefers wet, shady environments like gardens and forests. It is a herbivorous, nocturnal mollusc. With its ventral foot on a self-secreted track of mucus, it moves slowly. It features a long, dark grey body with a unique head with sensing tentacles and stalked eyes. On the dorsal side, the mantle creates a small shield-like area, resulting in a thin and reduced shell.



                                        Fig: Limax

Class Pelecypoda or Bivalvia

The exoskeleton in these animals possess two shells.Their feet are hatched or wedge-shaped, which aids in burrowing in the mud. Examples include Unio, Mytilus, Pinctada (Pearl oyster) etc. 



                                              Fig: Pinctada (Pearl oyster)

Unio 

The freshwater mussel is the common name of this animal. It is omnivorous. The glochidium larval stage, which lives as a parasite on fish, is part of the development process.



                                                 Fig: Unio

Class Cephalopoda

They are predators. The position of the shell is either external or internal. The brain in the head is quite large. The foot and head sections are united into a structure with eight to ten tentacles and several suction cups, bearing eyes. As a result of their head being below and tentacles being above while swimming, they are also known as 'Head over foot.' The mantle chamber is transformed into a device that allows water to be ejected with force to propel the animal forward like a jet. Some cephalopods may camouflage themselves by altering their skin colour to match the environment. Some members of this class include predators and they produce a mist of black ink-like substance. It is the only class in which the circulatory system is closed. Examples include Octopus, Sepia, Loligo, Nautilus etc. 



                                              Fig: Loligo (Squid)

Sepia

It is a good swimmer as well as a carnivore. It is commonly called cuttlefish. It has a shield-like trunk and a spherical head. A pair of large eyes and a ring of ten tentacles surround the mouth on the head. It hides from adversaries by changing colour to blend in with the surroundings or releasing a dark cloud of ink to elude them.




                             Fig: Sepia (Cuttlefish)

Octopus

The devil fish is its common name. It lives at the bottom of the sea in a store-built nest. It is a carnivore that uses its poisonous saliva to kill its prey. It may be educated to display manipulative skills and has a high learning capacity. They have 9 brains. Each of the eight arms has its own mini-brain in addition to the central brain, allowing it to act autonomously.



                                 Fig: Octopus

Economic importance of phylum Mollusca

  • Limax is a plant pest that puts a strain on seedlings, fragile branches, and leaves.
  • Unio is consumed by certain people as food.
  • Unio shells are used in the production of buttons, floor chips, and poultry feed.
  • The lime produced from Unio shells is of great quality.
  • Sepia is a mollusc that is used as food.
  • Artists make use of Sepia ink.
  • Oysters produce pearls, which are highly valued as gemstones.
  • Oysters can also be used as food.

Practice Problems

Q 1. Which of the following statements about molluscs is incorrect?

a. All have a soft segmented body that is divided into three parts: the head, the muscular foot, and the visceral mass
b. A mantle surrounds the visceral mass
c. A blood vascular system that is open can be observed
d. Molluscs are categorised as coelomates

Answer: Molluscs do not have a segmented body. Their body is differentiated into three regions like head, visceral hump and muscular foot. Molluscs are soft-bodied organisms with a calcareous shell that is protected by a soft, spongy covering known as the mantle. The visceral hump, also known as the visceral mass, is covered by the mantle. Molluscs have a body cavity lined by a mesoderm layer, hence they are coelomates. The circulatory system has open blood vessels or none at all. Hence the correct option is a.

Q 2. Match the following. 

Column I

Column II

A. Pila

I. Sea hare

B. Pinctada

II. Cuttlefish

C. Sepia

III. Apple snail 

D. Aplysia

IV. Pearl oyster


a. A - I, B - II, C - III, D - IV
b. A - IV, B - III, C - II, D - I
c. A - III, B - IV, C - II, D - I
d. A - III, B - IV, C - I, D - II

Answer: Pila is also known as the apple snail. The oyster Pinctada is responsible for pearl production. Cuttlefish is the name given to Sepia. Cuttlefish is a mollusc with an internal shell that lives in the sea. Aplysia is also known as the sea hare.

Hence the correct option is c.

Q 3. Molluscs are found only in which of the following animal groups?

a. Starfish, sea urchin and sea lily
b. Snail, devil fish and oysters
c. Crab, insects and spiders
d. Jellyfish, corals and sea pen

Answer: Snails, octopus (devil fish), and oysters are all members of the Mollusca phylum. The delicate, unsegmented bodies of these animals are protected by a protective shell. The exception is the devil fish or octopus, which lacks an outer protective shell. The phylum Echinodermata includes starfish, sea urchins, and sea lilies. The phylum Arthropoda includes crabs, insects, and spiders. Cnidaria includes jellyfish, corals, and sea pen. Hence the correct option is b.

Q 4. Molluscs differ from annelids in several ways. Find the right one from the options given below.

a. Having a closed circulatory system
b. Having direct development
c. Having a segmented body
d. Having an unsegmented body

Answer: Molluscs vary from annelids in that their bodies are not segmented. Annelids' bodies are separated into parts. Molluscs have an open circulatory system that is devoid of closed blood vessels. The tissues are directly bathed in blood. A closed circulatory system exists in annelids, with blood flowing through closed blood arteries. Development in annelids and molluscs is indirect and includes a larval stage. Hence the correct option is d.

FAQs

Q 1. Molluscs lay how many eggs?
Answer: An individual's egg production ranged from 2 to 25, although it may have been as high as 40. Almost all of the eggs deposited hatched in 30-45 days. Cannibalism of eggs by adult organisms has been seen by Elwell and Ulmer in them and it is also known to happen in other land snails too.

Q 2. What are bivalves and univalves?
Answer: Clams and oysters are bivalve molluscs with two shells. Animals with only one shell, such as snails, are known as univalves. Squid, banana slugs, and octopus are examples of molluscs without shells.

Q 3. Why do Molluscs lose their shells?
Answer: Total shell loss in terrestrial slugs could be an adaptation to cope with a lack of calcium. There is also evidence to prove that the original distribution of slugs were confined mainly to low-calcium environments.

Q 4. What is a devil’s toenail?
Answer: Gryphaea, often known as devil's toenails, is a genus of extinct oysters belonging to the Gryphaeidae family of marine bivalve molluscs. These fossils date from the Triassic period through the middle Paleogene period, with the Triassic and Jurassic being the most common. They are particularly prevalent in various places of the United Kingdom.

Q 5. Do snails have teeth?
Answer: Yes, snails have teeth. Despite the fact that their jaws are scarcely bigger than a pinhead, they can have over 25,000 teeth in their lifetime. All of which are positioned on the tongue and are constantly lost and replaced like shark teeth.

YOUTUBE LINK: https://www.youtube.com/watch?v=2V-RxvwschU&t=280s 

Related Topics 

Basis of Classification: Levels of organisation, Practice Problems and FAQs

Basis of Classification: Symmetry, Germ layer organisation, Coelom, Practice Problems and FAQs

Basis of Classification: Body plan, Segmentation, Notochord, Broad classification of Kingdom Animalia, Practice Problems and FAQs

Phylum Arthropoda, Practice Problems and FAQs

Phylum Echinodermata, Practice Problems and FAQs

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