Phylum Mollusca: Radula, Shell and the Second Largest Phylum
Biology · Animal Kingdom · NEET
Mollusca is the SECOND largest animal phylum (arthropods are first). A mollusc has a soft body covered by a calcareous shell, with three parts: a head, a muscular foot, and a visceral hump. Its most famous feature for NEET is the radula, a file-like rasping organ in the mouth used for feeding. Memory hook: "Molluscs Move on a foot, Munch with a radula, and wear a Mantle." Also remember: molluscs are coelomate but UNSEGMENTED.
A mollusc body has three parts: a head (with tentacles and a radula in the mouth), a muscular foot for movement, and a visceral hump covered by a mantle and a calcareous shell. Remember for NEET: molluscs are coelomate but UNSEGMENTED, the radula is unique to this phylum, and they have an open circulatory system.
Your doubts, answered
What exactly is the radula and which phylum has it?
The radula is a file-like, rasping organ found inside the mouth of molluscs. It is covered with rows of tiny hard teeth. The mollusc scrapes food (like algae) using the radula, the way a file scrapes wood. The radula is UNIQUE to Phylum Mollusca, so in matching questions, Radula always pairs with Mollusca. NEET example: Pila (apple snail) has a radula.
Is Mollusca the largest or the second largest phylum?
Mollusca is the SECOND largest animal phylum. The LARGEST is Phylum Arthropoda (insects, crabs). Students often mix these up. Remember the order: Arthropoda is 1st (largest), Mollusca is 2nd. NCERT states this directly, and NEET can ask it as a one-line fact.
Why is Mollusca a coelomate but still called unsegmented?
Coelom (a true body cavity lined by mesoderm) and segmentation are TWO separate features. Molluscs DO have a true coelom, so they are coelomates. But their body is NOT divided into repeating segments, so they are unsegmented. This is a classic trap: annelids and arthropods are coelomate AND segmented, but molluscs are coelomate and UNSEGMENTED. So if a question asks for coelomate + segmented animals, Mollusca must be excluded.
What is the mantle and the mantle cavity?
The mantle is a soft, spongy layer of skin that covers the visceral hump of the mollusc. The space between the visceral hump and the mantle is called the mantle cavity. Inside the mantle cavity are feather-like gills that do respiration (breathing) and excretion. So the mantle cavity is where a mollusc breathes.
What are the three body parts of a mollusc?
A mollusc body is unsegmented and has three clear parts: (1) a distinct HEAD (with sensory tentacles), (2) a muscular FOOT used for movement, and (3) a VISCERAL HUMP that holds the internal organs. The whole body is covered by a calcareous (calcium-containing) shell. Remember: Head, Foot, Visceral hump.
What are the common NEET examples of Mollusca?
Key NCERT examples: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia (sea hare), Dentalium (tusk shell), Chaetopleura (chiton). Yes, octopus and cuttlefish are molluscs, even though they look nothing like a snail.
⚠️ The NEET trap ✗ Choosing 'Annelida, Arthropoda and Mollusca' as the group that is organ-system level, bilaterally symmetrical, coelomate AND segmented. ✓ The correct group is Annelida, Arthropoda and Chordata. Mollusca is coelomate but UNSEGMENTED, so any option containing Mollusca is wrong. 🧠 Coelomate does NOT mean segmented. Mollusca has a true coelom but NO body segments, so cross out every option that lists Mollusca in a 'segmented' question.
Real NEET questions
NEET 2019
Match the following organisms with their respective characteristics:
Column I: A. Pila B. Bombyx C. Pleurobrachia D. Taenia
Column II: i. Flame cells ii. Comb plates iii. Radula iv. Malpighian tubules
A · A-iii, B-ii, C-i, D-iv
B · A-iii, B-iv, C-ii, D-i ✓
C · A-ii, B-iv, C-iii, D-i
D · A-iii, B-ii, C-iv, D-i
Solution: Pila is a mollusc, so it has a Radula (iii). Bombyx (silk moth, an arthropod) excretes through Malpighian tubules (iv). Pleurobrachia is a ctenophore with Comb plates (ii). Taenia is a flatworm that excretes with Flame cells (i). This gives A-iii, B-iv, C-ii, D-i, which is option (b).
NEET 2024
Match List I with List II:
List I: A. Pleurobrachia B. Radula C. Stomochord D. Air bladder
List II: I. Mollusca II. Ctenophora III. Osteichthyes IV. Hemichordata
A · A-II, B-I, C-IV, D-III ✓
B · A-II, B-IV, C-I, D-III
C · A-IV, B-III, C-II, D-I
D · A-IV, B-II, C-III, D-I
Solution: Pleurobrachia = Ctenophora (II). Radula is the file-like feeding organ of Mollusca (I). Stomochord is the notochord-like structure of Hemichordata (IV). Air bladder controls buoyancy in bony fishes, Osteichthyes (III). So A-II, B-I, C-IV, D-III, which is option (a).
NEET 2019
Consider following features:
(a) Organ system level of organisation
(b) Bilateral symmetry
(c) True coelomates with segmentation of body
Select the correct option of animal groups which possess ALL the above characteristics.
A · Annelida, Arthropoda and Chordata ✓
B · Annelida, Arthropoda and Mollusca
C · Arthropoda, Mollusca and Chordata
D · Annelida, Mollusca and Chordata
Solution: All three features (organ-system level, bilateral symmetry, true coelom + segmentation) are shown by Annelida, Arthropoda and Chordata. Mollusca is a true coelomate but is UNSEGMENTED, so every option containing Mollusca is wrong. Answer is option (a).
Solved Animal Kingdom NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Octopus (devil fish), cuttlefish (Sepia) and squid (Loligo) are all molluscs. They belong to Mollusca even though they do not have an obvious external shell like a snail.
Which is bigger, Arthropoda or Mollusca?
Arthropoda is the largest animal phylum. Mollusca is the second largest. Never swap this order in NEET.
Do molluscs have an open or closed circulatory system?
Molluscs have an OPEN circulatory system, the same as arthropods. Blood flows into open spaces called sinuses instead of staying inside closed vessels.
What is the level of organisation in Mollusca?
Molluscs have an organ-system level of organisation. They are also bilaterally symmetrical, triploblastic and coelomate.
Is Pila a mollusc?
Yes. Pila (the apple snail) is a standard NCERT example of Mollusca and is famous for having a radula.
