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Phylum Mollusca

Introduction:

  • It is considered as the second largest phylum of kingdom animalia.
  • It includes about 60,000 species.
  • Study of molluscs is known as Malacology.
  • Study of shells of molluscs is known as Conchology.

Topics covered

  • General characteristics of phylum Mollusca
  • Classification of phylum Mollusca
  • Examples

General characteristics of phylum Mollusca
 

Detailed explanation:
 

Habit and habitat

  • Mostly marine.
  • Some are terrestrial found in the damp soil.

Level of organisation

  • Organ-system level of organisation.

Symmetry

  • Bilateral symmetry.
  • Members of class Gastropoda are asymmetrical (eg : Snail) due to the presence of Torsion during growth.

Germ layers

  • These are triploblastic animals i.e the body consists of three germ layers.

Body

  • Their body is unsegmented and divided into three parts -
  • Head - includes sense organs like eyes and sensory tentacles.
  • Dorsal visceral hump - includes all visceral organs of the body.
    - Visceral hump is covered with a soft and spongy layer called a mantle or pallium.
    - Mantle cavity is present between the visceral hump and mantle in which feather-like gills are found.
    - Their body is covered by a shell which is secreted by a mantle.
    - Their shell is made up of calcium carbonate and conchiolin protein.
  • Ventral muscular foot for locomotion.

Body plan

  • Tube-within-tube body plan.

Body wall

  • Their body wall consists of a single layered epidermis that is usually ciliated.
  • Muscles are usually stripped and occur in bundles.

Coelom (body cavity)

  • Eucoelomates i.e cavity is present but it is reduced.
  • Schizocoelomates
  • The spaces between the viscera is filled with the blood forming haemocoel.

Digestion

  • Digestive system is complete.
  • Their buccal cavity contains a file-like rasping structure having transverse rows of teeth for feeding and known as radula.
  • Their anus opens into the mantle cavity.

Respiration

  • Respiration takes place by feather-like gills called ctenidia.
  • Respiration in land forms like Pila takes place by pulmonary sac while in water it occurs by gills .

Excretion

  • Excretory system includes a paired Kebers organ or organ of Bojanus that opens into the mantle cavity.
  • Feather-like gills also help in excretion in some molluscs.
  • Ammonia is the nitrogenous waste in aquatic form while uric acid is the nitrogenous waste in land or terrestrial form.

Blood vascular system or circulatory system

  • Open type circulatory system is present.
  • Heart is dorsally present and it opens into the sinuses (spaces).
  • Blood is usually blue due to the presence of copper containing blue respiratory pigment called haemocyanin.

Nervous system

  • Nervous system consisting of the paired cerebral, pedal and visceral ganglia joined by nerve connectives and commissures.

Sense organs

  • Sensory organs are present, like
    - In some molluscs, eyes are present over stalks called ommatophore
    - Statocyst, balancing organ is present
    - In some molluscs, osphradium is present for testing the physical and chemical nature of water.

Reproductive System

  • Reproduction is only sexual .

Development

  • Development is indirect, in which the larval stages like veliger, trochophore or glochidium are formed.
  • They show metamorphosis during indirect development.

Special characteristics of phylum Mollusca

  • Presence of mantle that covers the body
  • Presence of shell made up of calcium carbonate
  • Presence of radula in their buccal cavity

Classification of phylum Mollusca
 

Detailed explanation:
 

Mollusca is divided into six classes on the basis of shell and foot

1. Monoplacophora

  • Their shell is either spoon or cup-shaped.
  • Example : Neopilina.

2. Polyplacophora

  • A non ganglionated nerve ring around the mouth with two pairs of interconnected nerve cord is present in them.
  • Shell has multiple plates.
  • Example - Chaetopleura (Chiton)

3. Scaphopoda

  • Their shell is tubular and open at both ends.
  • Example : Dentalium (Tusk shell)

4. Gastropoda

  • Their shell is made up of one piece.
  • The early embryo is symmetrical but during development their shell becomes spirally coiled showing torsion so that the body becomes asymmetrical.
  • It includes the largest number of molluscs
  • Example - Pila, Aplysia (sea hare), Doris (sea lemon), Limnaea, (pond snail), Planorbis, Patella (true limpet), Turbinella (Shankh), .

5. Pelecypoda or Bivalvia

  • Their shell is made up of two halves.
  • Examples : Unio (freshwater mussel), Mytilus (Sea mussel), Teredo (shipworm), Ostrea (edible oyster), Pinctada (Pearl oyster).

6. Cephalopoda

  • Their head and foot regions are combined and modified into a structure which has eyes and eight tentacles, hence the name cephalopod or 'head foot'.
  • Cephalopods are regarded at the top of invertebrates evolution in terms of learned behaviour they exhibit.
  • Some cephalopods are the largest invertebrates.
  • Nautilus (shell is external)
  • Octopus (shell is absent)
  • Sepia (shell is internal)
  • Examples : Sepia, Loligo, Octopus, Nautilus (Pearly nautilus).

Examples
 

Detailed explanation:
 

1. Neopilina

  • It is a living fossil that is the connecting link between phylum Annelida and phylum Mollusca as they are having the character of both phyla

2. Chaetopleura (Chiton)

  • They are commonly known as “The Coat of Mail Shell”
  • Chiton is marine and found attached to rocks by its foot.

3. Dentalium

  • They are commonly known as “The Elephant's tusk-shell”.
  • They respire through mantle.

4. Unio

  • They are commonly known as “The Fresh Water Mussel”.
  • It is an aquatic animal which is found in rivers, lakes and ponds.
  • It acts as a scavenger and keeps water clean. Its shell yields an excellent quantity of lime.
  • The shells of the freshwater mussels are used in the manufacture of buttons.

5. Pila

  • They are commonly known as “Apple snail”.
  • They are herbivorous and feeds on aquatic plants like Pistia and Vallisneria.

    pila
     

6. Limax

  • They are commonly known as “Grey Slug” and the slug is a plant pest.
  • Slowest Invertebrate.
  • It is terrestrial and is abundantly found in gardens, cultivated lands and over damp soil.

7. Sepia

  • They are commonly known as “Cuttle fish”. moke cloud.
  • Their shell is used as a source of calcium for pet birds.

8. Loligo

  • They are commonly known as “Squid”.
  • They has an internal shell.
  • They are edible.
  • Largest living invertebrate is a giant squid (Architeuthis) upto 18 meters long.
  • The giant squid has the largest eye in the animal kingdom.

9. Octopus

  • They are commonly known as “Devil fish”.
  • They kill its prey with poisonous saliva.
  • They eject an inky fluid in water and form a screen for defence from its enemies.
  • They can change its colour.

    octopus
     

Frequently Asked Questions: FAQs
 

Q1. Which is the second largest phylum of Kingdom Animalia ?
Ans :

  • Mollusca is the second largest phylum of Kingdom Animalia.

Q2. What is the study of molluscs called?
Ans :
Malacology

Q3. Why are the members of class Gastropoda asymmetrical ?
Ans :

  • Members of class Gastropoda are asymmetrical due to the presence of Torsion during growth.
  • Example - Snail

Q4. Which is the respiratory organ of the animals belonging to phylum Mollusca ?
Ans :

  • The respiratory organ of the animals belonging to phylum Mollusca is feather-like gills called ctenidia.
  • Respiration in land forms like Pila takes place by pulmonary sac while in water it occurs by gills .

Q5. Which is the excretory organ of the animals belonging to phylum Mollusca ?
Ans :

  • The Excretory organ of the animals belonging to phylum Mollusca is Kebers organ or Organ of Bojanus that opens into the mantle cavity.

Q6. Describe the structure of typical Molluscs.
Ans :

  • Their body is unsegmented and divided into three parts -
  • Head - includes sense organs like eyes and sensory tentacles.
  • Dorsal visceral hump - includes all visceral organs of the body.
  • Ventral muscular foot for locomotion.

Q7. Why is the blood of Molluscs, blue coloured ?
Ans :

  • Blood is usually blue due to the presence of copper containing blue respiratory pigment called haemocyanin.

Q8. Which is the connecting link between phylum Annelida and Mollusca ?
Ans :

  • Neopilina is a living fossil that is the connecting link between phylum Annelida and phylum Mollusca as they are having the character of both phyla.

Q9. What are the larval stages of phylum Mollusca ?
Ans :

  • The larval stages of phylum Mollusca are veliger, trochophore or glochidium.

Q10. Which is the largest living invertebrate ?
Ans :

  • The largest living invertebrate is a giant squid (Architeuthis) upto 18 meters long.
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