Mention the characteristics of phylum Mollusca.

**OR**

Draw a labelled diagram of human eye. Differentiate between rods and cones.

Question

Mention the characteristics of phylum Mollusca.

**OR**

Draw a labelled diagram of human eye. Differentiate between rods and cones.

Accepted Answer

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> [!TLDR]
> Mollusca are soft-bodied, triploblastic, coelomate animals with a head-foot-visceral-hump body plan, a mantle secreting a calcareous shell, and (in most) a radula for feeding.
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> This is an either/or question; the primary part is answered here.
>
> Phylum Mollusca is the second largest animal phylum after Arthropoda, and includes animals such as snails (Pila), mussels, oysters (Pinctada), and octopus. Its key diagnostic characteristics are:
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> 1. Body organisation — triploblastic, bilaterally symmetrical (though some, like gastropods, become asymmetrical due to a developmental process called torsion), and coelomate (possessing a true body cavity, though reduced).
> 2. Body divisions — the soft body is organised into a distinct head, a ventral muscular foot (used for locomotion, and modified into tentacles in cephalopods), and a visceral hump (containing the internal organs).
> 3. Mantle — a soft, fleshy fold of skin called the mantle covers the visceral hump, enclosing a space called the mantle cavity (which usually houses the gills); in most molluscs, the mantle also secretes a hard, calcareous (calcium-carbonate-based) shell that protects the soft body.
> 4. Respiration — mostly through gills, called ctenidia, located in the mantle cavity, though some terrestrial forms (like land snails) use a vascularised mantle wall as a lung.
> 5. Circulatory system — generally an open type (except in the more advanced cephalopods, such as octopus and squid, which have a closed circulatory system).
> 6. Excretion — carried out by kidneys (metanephridia).
> 7. Feeding — most molluscs (except bivalves) possess a unique rasping, file-like feeding organ called the radula, used to scrape food.
> 8. Reproduction — molluscs are usually unisexual (dioecious), and development may include a distinctive larval stage. …

Feature	Description
Body symmetry	Usually bilateral (some gastropods show torsion, making it asymmetrical)
Coelom	Reduced, mostly around the heart and gonads
Digestive system	Complete (mouth → anus), with a radula in most
Circulatory system	Open (except cephalopods, which have a closed system)
Respiration	Gills (ctenidia) in aquatic forms; a lung-like mantle cavity in terrestrial snails
Excretion	One or two nephridia (kidney-like organs)
Nervous system	Ganglia connected by nerve cords; sense organs (eyes, tentacles) well developed in cephalopods
Reproduction	Mostly sexual, with external or internal fertilisation; many have a free-swimming larval stage (trochophore or veliger)

Class	Common name	Key features	Example
Gastropoda	Snails, slugs	Asymmetrical body due to torsion; one-piece coiled shell (or absent); head with tentacles and eyes	Pila (apple snail), Helix (garden snail)
Bivalvia	Clams, oysters, mussels	Two hinged shells; no radula; filter feeders; foot is hatchet-shaped for burrowing	Unio (freshwater mussel), Mytilus (marine mussel)
Cephalopoda	Squid, octopus, cuttlefish	Shell reduced or absent; foot modified into arms/tentacles; head well developed with large eyes; closed circulatory system; jet propulsion	Octopus, Sepia (cuttlefish), Loligo (squid)
Polyplacophora	Chitons	Eight overlapping shell plates; foot broad and flat; radula present; marine, cling to rocks	Chiton

Q.Mention the characteristics of phylum Mollusca. OR Draw a labelled diagram of human eye. Differentiate between rods and cones.

Concept understanding — Phylum Mollusca

Phylum Mollusca – From Intuition to Precision

The Three-Part Body Plan

The Radula – A Unique Feeding Tool

Key Features at a Glance

The Major Classes You Need to Know

Q.Mention the characteristics of phylum Mollusca. OR Draw a labelled diagram of human eye. Differentiate between rods and cones.

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