The phylum Mollusca represents one of the most diverse groups of invertebrates. They include familiar animals such as snails, clams, squids, and octopuses. Molluscs occupy marine, freshwater, and terrestrial habitats.
General Characteristics of Mollusca
1. Body Organization: Molluscs are triploblastic, bilaterally symmetrical, and coelomate animals with organ-system level of organization.
2. Body Plan: The body is soft, unsegmented, and divisible into three main regions: head-foot (used for locomotion and sensory perception), visceral mass (containing most internal organs), and mantle (a specialized fold that secretes the shell).
3. Symmetry: Most molluscs exhibit bilateral symmetry, though torsion in gastropods leads to asymmetry.
4. Shell: Many molluscs possess a calcareous shell secreted by the mantle. In some, the shell is reduced or absent (e.g., octopus, slugs).
5. Coelom: They have a true coelom, which allows space for organ development and efficient circulation.
6. Digestive System: The alimentary canal is complete. Most classes possess a rasping organ called radula for scraping food, except in bivalves.
7. Circulatory System: Circulation is usually open (e.g., in bivalves and gastropods), but in cephalopods it is closed, enabling high activity levels.
8. Respiration: Respiration occurs through paired gills called ctenidia, though in terrestrial forms it may occur through a lung-like cavity.
9. Excretion: Excretion takes place via nephridia, particularly metanephridia, which open into the mantle cavity.
10. Nervous System: The nervous system ranges from simple (in chitons) to highly advanced (in cephalopods with complex brains and eyes).
11. Reproduction: Molluscs are usually dioecious (separate sexes), with both external and internal fertilization observed depending on class.
12. Development: Development often includes free-swimming larval forms such as trochophore and veliger larvae.
Classification of Mollusca up to Class
The phylum Mollusca is a diverse group of soft-bodied invertebrates. They are classified into different classes on the basis of Structure and presence of shell, Type of foot and its modification, Presence or absence of radula and Mode of feeding and lifestyle.
The major classes of Mollusca are described below:
1. Class Monoplacophora
I. These are considered the most primitive molluscs.
II. The body is bilaterally symmetrical and covered with a simple, cap-like shell.
III. A key feature is the serial repetition of organs such as gills, nephridia, and muscles, which gives them a resemblance to annelids.
IV. They are mostly marine and deep-sea dwelling.
Example: Neopilina (considered a living fossil).
2. Class Polyplacophora
i. Commonly called chitons.
ii. The body is dorsoventrally flattened and protected by a dorsal shell composed of eight articulating plates.
iii. The ventral side bears a broad, flat foot used for strong adhesion to rocks in intertidal zones.
iv. They are marine, herbivorous grazers that feed using a radula to scrape algae off rock surfaces.
Example: Chiton.
3. Class Aplacophora
I. These are worm-like, cylindrical molluscs that lack a shell.
II. Instead of a shell, their body surface is covered with minute calcareous spicules.
III. They are mostly found in deep-sea habitats where they burrow into sand or mud.
IV. They feed on small invertebrates or detritus.
Example: Chaetoderma, Neomenia.
4. Class Scaphopoda
i. These molluscs are known as tusk shells due to their elongated, tubular shell that is open at both ends.
ii. They live buried in sand with the smaller end protruding for water circulation.
iii. They feed on microscopic organisms and detritus using tentacle-like captacula.
iv. They are exclusively marine forms.
Example: Dentalium.
5. Class Bivalvia
I. Includes clams, mussels, oysters, and scallops.
II. The body is laterally compressed and enclosed by two calcareous valves (shells) joined by a hinge.
III. They lack a radula and are specialized filter feeders, drawing in water to trap plankton and organic matter using gills.
IV. The foot is laterally compressed and adapted for burrowing.
V. Many are of economic importance, providing food and pearls (Pinctada).
VI. Examples: Unio (freshwater mussel), Mytilus (marine mussel), Pinctada (pearl oyster), Ostrea (edible oyster).
6. Class Gastropoda
i. This is the largest class of Mollusca, including snails, limpets, slugs, and whelks.
ii. They undergo a unique process called torsion, where the visceral mass twists 180° during larval development, resulting in asymmetry.
iii. Most possess a coiled shell, though in some (e.g., slugs) the shell is reduced or absent.
iv. They have a distinct head with tentacles and eyes, and a muscular foot for crawling.
v. They are highly diverse, living in marine, freshwater, and terrestrial habitats.
Examples: Pila (apple snail), Patella (limpet), Limax (slug), Murex (rock shell).
7. Class Cephalopoda
I. These are the most highly advanced molluscs.
II. Body is bilaterally symmetrical with a head-foot modified into tentacles (bearing suckers) surrounding the mouth.
III. They possess a beak-like jaw for feeding, and most have an ink gland for defense.
IV. The circulatory system is closed, enabling fast activity and predation.
V. Nervous system is highly developed; cephalopods are regarded as the most intelligent invertebrates with well-developed eyes.
VI. Many exhibit jet propulsion by expelling water through a siphon.
Examples: Octopus, Loligo (squid), Sepia (cuttlefish), Nautilus (chambered nautilus).
Frequently Asked Questions (FAQs)
Q: What is the economic importance of molluscs?
A: Molluscs are a source of food (clams, oysters, squids), pearls, shells, and bioindicators. Some are agricultural pests, while others serve in medical research.
Q: Which molluscs are considered intelligent?
A: Cephalopods such as octopus, cuttlefish, and squid show remarkable intelligence, problem-solving ability, and complex behavior.
Q: Do all molluscs live in water?
A: No. While most are marine, some gastropods are freshwater or terrestrial.
Q: What larval stages are seen in molluscs?
A: Trochophore and veliger larval stages are typical in many molluscan groups.
Multiple Choice Questions (MCQs)
1. Which structure is absent in Bivalvia?
a) Gills
b) Mantle
c) Radula
d) Nephridia
Answer: c) Radula
2. Which molluscan class exhibits torsion?
a) Bivalvia
b) Gastropoda
c) Cephalopoda
d) Polyplacophora
Answer: b) Gastropoda
3. The shell of chitons consists of:
a) One piece
b) Two valves
c) Eight plates
d) Absent
Answer: c) Eight plates
4. Which class has a closed circulatory system?
a) Gastropoda
b) Cephalopoda
c) Bivalvia
d) Scaphopoda
Answer: b) Cephalopoda
5. The locomotory organ of most molluscs is:
a) Tentacles
b) Foot
c) Parapodia
d) Fins
Answer: b) Foot
Worksheet for Practice
1. Draw a neat labelled diagram of a generalized mollusc.
2. Explain the process of torsion in gastropods with a diagram.
3. Compare the circulatory systems of bivalves and cephalopods.
4. Discuss the adaptive significance of the shell in molluscs.
5. Write short notes on: (a) Radula (b) Ctenidia (c) Mantle cavity.
6. Classify Mollusca up to class with suitable examples.
7. Give the economic importance of molluscs with examples.
.png)
Posts
0 Comments