Molluscs: Overview | Zoology Optional Notes for UPSC PDF Download

Introduction

Mollusca is a diverse phylum encompassing soft-bodied, triploblastic, bilaterally symmetrical, and coelomate animals. The study of Mollusca is known as Malacology. These invertebrates, characterized by a thin fleshy mantle enveloping visceral organs, play crucial roles in human life, serving as sources of both jewelry and food. Natural pearls, formed within mollusks, are notable products.

Etymology

The term "Mollusca" originates from Aristotle's designation for cuttlefish, with "Mollusc" signifying softness.

Habitat and Size

Mollusks are found in terrestrial and deep-sea environments, ranging from microscopic organisms to those as large as 20 meters. Their ecological importance extends to bioindicators in marine and freshwater ecosystems, albeit some, like snails and slugs, are considered pests.

Mollusca Characteristics

1. Habitat

• Predominantly marine and freshwater; some are terrestrial.
• Few terrestrial species found in moist soil.

2. Organ System Level of Organization

• Exhibits organ system level organization.

3. Body Structure

• Head, visceral mass, muscular foot, and mantle constitute the body.
• Head features tentacles and compound eyes.

4. Shell

• Covered by a calcareous shell.

5. Locomotion

• Muscular foot facilitates locomotion.

6. Digestive System

• Well-developed digestive system.
• Radula serves as the rasping organ for feeding.

7. Respiration

• Respiration occurs through the general body surface, gills, or pulmonary sac.

8. Circulatory System

• Open circulatory system for blood circulation.

9. Excretion

• Pair of metanephridia aid in excretion.

10. Nervous System

• Comprises paired ganglia and nerves.
• Sensory organs include tentacles, eyes, osphradium, and statocysts.

11. Reproduction

• Most species exhibit separate sexes; some are hermaphrodites.
• Fertilization may be external or internal.
• Generally oviparous with indirect development.

Classification of Mollusca

1. Aplacophora or Solenogasters

• Cylindrical or bilaterally symmetrical body.
• Lack head, shell, mantle, nephridia, and foot.
• Dorsal longitudinal keel or crest present.
• Examples: Neomenia, Chaetoderma.

2. Monoplacophora

• Bilaterally symmetrical body.
• Head lacks eyes and tentacles.
• Respiration via externally located gills.
• Excretion through nephridia.
• Example: Neopilina.

3. Polyplacophora

• Dorsoventrally flattened body with 8 longitudinal plates.
• Well-developed radula.
• Flat ventral foot.
• Examples: Chiton, Cryptochiton.

4. Gastropoda

• Found in various environments.
• Head bears tentacles, eyes, and mouth.
• Spiral-shaped shell.
• Large, flat foot.
• Examples: Haliotis, Pila.

5. Scaphopoda

• Marine environment inhabitants.
• Absence of eyes and tentacles.
• Reduced foot.
• Bilaterally symmetrical body.

6. Pelecypoda

• Aquatic habitat residents.
• Bilaterally symmetrical, laterally compressed body.
• Lack of distinct head.
• Burrow in mud and sand.
• Examples: Mussels, Unio.

7. Cephalopoda

• Predominantly marine.
• Shell may be external, internal, or absent.
• Separate sexes with direct development.
• Examples: Octopus, Spirula.

Mollusca Examples

A selection of Mollusca examples includes Pila, Limax, Unio, Turbinella, Helix, Octopus, and Loligo.

Key Points on Mollusca

• Phylum Mollusca ranks as the second-largest phylum.
• Some mollusks, like Unio, possess green glands mimicking vertebrate livers.
• Osmoregulation facilitated by osphradia.
• Statocysts maintain body equilibrium.
• Octopus, lacking a shell, has ink glands for protection.
• Blue or green blood results from the copper-containing pigment haemocyanin.
• Possession of a myogenic heart.

In summary, the phylum Mollusca showcases diverse organisms with unique characteristics and ecological significance, contributing to various aspects of human life.