Segmentation means the animal's body is divided into a repeating series of segments, both outside and inside. In the earthworm, some organs repeat in each segment. This pattern is called metameric segmentation, and the phenomenon itself is called metamerism. Memory hook: think of a train made of many similar coaches joined in a line, each coach = one segment (metamere).
Metameric segmentation in the earthworm: the body is divided into a serial line of repeating segments (metameres), with organs repeating in each. True metamerism is seen in Annelida, Arthropoda and Chordata; Mollusca is a coelomate but stays unsegmented.
Your doubts, answered
What exactly is metameric segmentation?
It means the body is divided, both on the outside and the inside, into a line of segments called metameres. Some organs (like nephridia, nerve ganglia, blood vessels) repeat in each segment. NCERT gives the earthworm as the example: its body shows serial repetition of at least some organs, so it is metamerically segmented.
Is metamerism the same thing as segmentation? Why two words?
They point to the same idea but are used slightly differently. 'Segmentation' is the general fact that the body is split into segments. 'Metamerism' (or metameric segmentation) is the special true form where the segments are repeated in a serial way, both outside and inside, with organs repeating. For NEET, treat metamerism = true segmentation seen in Annelida, Arthropoda and Chordata.
Which phyla show metameric segmentation for NEET?
Annelida (e.g. earthworm, Nereis, leech), Arthropoda (e.g. cockroach, prawn) and Chordata all show true metameric segmentation. NCERT even names Annelida after this feature: 'annulus' means little ring, and the body is marked into segments called metameres.
Why is Mollusca NOT segmented even though it is a coelomate?
Mollusca has a true coelom, but its body is unsegmented. NCERT clearly says the molluscan body is 'unsegmented with a distinct head, muscular foot and visceral hump.' This is a very common NEET trap: coelom and segmentation are two separate features, so an animal can have a coelom yet still be unsegmented.
Is the tapeworm (proglottids) an example of true metamerism?
No. In a tapeworm the body is broken into proglottids, but this is called pseudometamerism (false segmentation), not true metamerism. NCERT reserves true metameric segmentation for the earthworm/Annelida pattern where organs repeat in each segment. Do not call flatworm proglottids true metamerism in NEET.
Where is segmentation placed among the basis of classification?
NCERT lists segmentation as one of the fundamental features used to classify the animal kingdom, along with levels of organisation, symmetry, germ layers, coelom and notochord. So examiners can test it as a stand-alone character or mix it into a match/statement question.
⚠️ The NEET trap ✗ Mollusca is a coelomate, so it must also be metamerically segmented. ✓ Mollusca has a true coelom but is UNSEGMENTED. True metamerism is seen in Annelida, Arthropoda and Chordata, not Mollusca. 🧠 Coelom and segmentation are two different tick-boxes. A coelomate can still be unsegmented — Mollusca proves it.
Real NEET questions
2019
Consider the 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 must be true together. Annelida, Arthropoda and Chordata are organ-system level, bilaterally symmetrical, true coelomate AND metamerically segmented. Mollusca is a true coelomate but is UNSEGMENTED, so every option that contains Mollusca is wrong. That removes B, C and D, leaving option A.
2021
Match List I with List II. List I: (a) Metamerism (b) Canal system (c) Comb plates (d) Cnidoblasts. List II: (i) Coelenterata (ii) Ctenophora (iii) Annelida (iv) Porifera.
A · (a)-(iii); (b)-(iv); (c)-(ii); (d)-(i) ✓
B · (a)-(iv); (b)-(i); (c)-(ii); (d)-(iii)
C · (a)-(iv); (b)-(iii); (c)-(i); (d)-(ii)
D · (a)-(iii); (b)-(iv); (c)-(i); (d)-(ii)
Solution: Metamerism (metameric segmentation) is the defining feature of Annelida, so (a)-(iii). Canal system is found in Porifera, so (b)-(iv). Comb plates belong to Ctenophora, so (c)-(ii). Cnidoblasts (stinging cells) define Coelenterata, so (d)-(i). This matches option A.
2016
Which of the following features is not present in the Phylum Arthropoda?
A · Chitinous exoskeleton
B · Metameric segmentation
C · Parapodia ✓
D · Jointed appendages
Solution: Arthropods do have a chitinous exoskeleton, metameric (segmented) body and jointed appendages, so those are wrong choices for a 'not present' question. Parapodia are lateral appendages of aquatic annelids like Nereis, not arthropods. Note this question confirms that Arthropoda IS metamerically segmented, so option C is the correct answer.
Solved Animal Kingdom NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes, for NEET treat metameric segmentation and metamerism as the same true-segmentation feature, first clearly seen in Annelida such as the earthworm.
Name three phyla that are metamerically segmented.
Annelida, Arthropoda and Chordata all show true metameric segmentation. Mollusca is a coelomate but stays unsegmented.
What is a metamere?
A metamere is one single segment of a segmented body. In the earthworm the body is marked into many such rings called metameres, giving the phylum its name Annelida (annulus = little ring).
Which animal does NCERT use as the example of metamerism?
NCERT uses the earthworm. Its body is divided outside and inside into segments with serial repetition of at least some organs, which is exactly metameric segmentation.
Can an animal have a coelom but no segmentation?
Yes. Mollusca is the classic example: it has a true coelom but its body is unsegmented. Coelom and segmentation are two independent features.
