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Zoology · Animal Kingdom

Segmentation & Notochord

Segmentation and the notochord are the last two fundamental criteria NCERT lists before it builds the broad classification of the animal kingdom. One describes the serial repetition of body parts; the other defines a dorsal rod that separates chordates from non-chordates. NEET tests both almost every year through statement, assertion–reason and match-the-column questions, so the metameric-segmentation rule and the chordate-defining role of the notochord must be exam-ready.

NCERT grounding

This subtopic is anchored in NCERT Class 11 Biology, Chapter 4 Animal Kingdom, where Section 4.1.5 covers Segmentation and Section 4.1.6 covers the Notochord. Both sit inside Section 4.1, "Basis of Classification", alongside levels of organisation, symmetry, the diploblastic and triploblastic plan, and the coelom. NCERT treats these features as the small set of fundamental characters that, taken together, allow more than a million animal species to be sorted into phyla.

NCERT defines segmentation directly: "In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs." It then names the earthworm as the example and calls the pattern metameric segmentation, with the phenomenon itself termed metamerism. The notochord definition is equally compact: "Notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals." Animals that form it are chordates; those that do not are non-chordates, from Porifera to Echinodermata.

"The basic fundamental features such as level of organisation, symmetry, cell organisation, coelom, segmentation, notochord, etc., have enabled us to broadly classify the animal kingdom."

Because NCERT lists segmentation and the notochord side by side as classification criteria, this page treats them together. Both appear explicitly as columns in NCERT Table 4.2, "Salient Features of Different Phyla", and the same table is the source of every "present/absent" verdict used below.

Two criteria, one classification

Segmentation and the notochord answer two different structural questions, yet NCERT places them at the same point in the chapter for the same reason: each one, on its own, separates a small set of advanced phyla from the rest of the animal kingdom. Segmentation asks whether the body is built from a series of repeated units. The notochord asks whether a dorsal supporting rod ever appears during development. A NEET candidate who can apply both rules can place any phylum on the classification tree without memorising the whole table.

The two criteria are independent of each other but they are not independent of the other classification features. Both segmentation and the notochord appear only in animals that are bilaterally symmetrical, triploblastic and coelomate. NCERT's own ordering is deliberate: symmetry, germ layers and coelom are introduced first because they describe broad grades of body organisation, and only then do segmentation and the notochord refine those grades further.

Read this first. Segmentation is about the repetition of body parts along the long axis. The notochord is about a single dorsal rod derived from mesoderm. They are separate criteria — an animal may have one, both or neither.

Segmentation

Body externally and internally divided into segments with serial repetition of at least some organs.

Phenomenon = metamerism; the pattern = metameric segmentation.

NCERT example: earthworm

Notochord

A mesodermally derived rod formed on the dorsal side during embryonic development.

Animals with it = chordates; without it = non-chordates (Porifera to Echinodermata).

Defines phylum Chordata

Segmentation and metamerism

In an unsegmented animal such as a roundworm or a snail, the body is a single continuous unit. In a segmented animal the body is instead a stack of repeated modules. NCERT's phrasing carries two demands that NEET tests carefully. First, the division must be external and internal — surface rings alone are not enough; the internal organs must repeat too. Second, there must be a serial repetition of at least some organs — the word "some" is important, because not every organ is repeated, only a recurring subset.

The earthworm is NCERT's chosen illustration. Its body surface is "distinctly marked out into segments or metameres", which is why the phylum is named Annelida, from the Latin annulus, a little ring. Inside the earthworm, structures such as the nephridia and the segmental nerve ganglia repeat from segment to segment, satisfying the internal half of the definition. This combination — external rings plus internally repeated organs — is what NCERT calls true metameric segmentation, and it is the standard against which other phyla are judged.

Metamerism is not merely a labelling convenience. Each segment houses its own set of muscles, and the annelid's longitudinal and circular muscles act segment by segment, which is what allows the coordinated, wave-like burrowing movement of the earthworm. Segmentation therefore couples a developmental pattern to a functional advantage, although NEET, following NCERT, examines mainly the definition and the phylum list rather than the mechanics.

Figure 1 Unsegmented vs metamerically segmented body Unsegmented body e.g. roundworm — single continuous unit no serial repetition of organs Metamerically segmented body e.g. earthworm — repeated metameres, organs repeat segment by segment spacer

Figure 1. In an unsegmented body the internal organs do not repeat along the long axis. In a metamerically segmented body the body wall is divided into metameres and a subset of organs (shown as repeated discs) recurs in each segment — the "serial repetition of at least some organs" NCERT specifies.

Segmentation across the phyla

NCERT Table 4.2 records segmentation as Present in exactly three phyla — Annelida, Arthropoda and Chordata — and as Absent in the other eight: Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Mollusca, Echinodermata and Hemichordata. This three-phylum list is one of the most directly testable facts in the chapter.

PhylumSegmentationNCERT note
AnnelidaPresentBody marked into segments or metameres; "body segmentation like rings".
ArthropodaPresentBilaterally symmetrical, triploblastic, segmented and coelomate.
ChordataPresentListed as "Present" in Table 4.2 alongside the notochord characters.
MolluscaAbsentNCERT states the body is "unsegmented with a distinct head, muscular foot and visceral hump".
AschelminthesAbsentRoundworms — circular in cross-section, no segments.
PlatyhelminthesAbsentFlatworms — dorso-ventrally flattened, unsegmented.

NCERT highlights Annelida as the phylum where this feature is the diagnostic signature: the name itself means "little ring". Arthropoda is also "segmented" in NCERT's wording, with the body organised into head, thorax and abdomen and carrying jointed appendages — the segmental plan is still present, although the segments are grouped into functional regions. Chordata appears as "Present" in Table 4.2 as well, which is the verdict NEET expects candidates to reproduce.

An important point of order arises from NCERT exercise question 13: "Segmentation in the body is first observed in which of the following: (a) Platyhelminthes (b) Aschelminthes (c) Annelida (d) Arthropoda." The intended answer is Annelida. Reading the phyla in the sequence NCERT presents them, Annelida is the first phylum in which segmentation appears, so it marks the evolutionary debut of the metameric body plan within the chapter's framework.

Two phyla, two faces of segmentation

Annelida

Metameres

uniform repeated rings

  • Body surface "distinctly marked out into segments or metameres".
  • Name from Latin annulus = little ring.
  • Longitudinal and circular muscles act segment by segment.
  • Nephridia repeat as segmental excretory organs.
vs

Arthropoda

Tagmata

segments grouped into regions

  • Still "segmented" in NCERT Table 4.2.
  • Body consists of head, thorax and abdomen.
  • Jointed appendages — arthros = joint, poda = appendages.
  • Covered by a chitinous exoskeleton.

One trap to anticipate: NEET 2016 asked which feature is not present in Arthropoda, with "metameric segmentation" among the options. The correct answer there was parapodia — metameric segmentation is a feature of arthropods. Parapodia are the lateral swimming appendages of polychaete annelids such as Nereis, not of arthropods. So segmentation belongs to arthropods; parapodia do not.

The notochord

The notochord is NCERT's single defining criterion for the phylum Chordata. By definition it is "a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals." Three parts of that sentence are individually examinable. It is mesodermally derived — it originates from the middle germ layer, not from ectoderm or endoderm. It is rod-like and lies on the dorsal side of the body. And it forms during embryonic development — it is a developmental structure, not necessarily a permanent adult one.

1

One structure splits the kingdom

Animals that form a notochord are chordates; animals that never form one are non-chordates — NCERT names Porifera through Echinodermata as the non-chordate phyla.

The notochord never appears alone. NCERT describes the chordate body plan as a package of three fundamental characters: a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits. Chordates are additionally bilaterally symmetrical, triploblastic and coelomate with organ-system level of organisation, and they possess a post-anal tail and a closed circulatory system with a ventral heart. NCERT's Table 4.1 lays out the contrast with non-chordates directly.

FeatureChordatesNon-chordates
NotochordPresentAbsent
Central nervous systemDorsal, hollow and singleVentral, solid and double
Pharyngeal gill slitsPharynx perforated by gill slitsGill slits absent
HeartVentralDorsal (if present)
Post-anal tailPresentAbsent

The hemichordate is the structure NCERT uses to test whether students truly understand the notochord. Hemichordata "was earlier considered as a sub-phylum under phylum Chordata. But now it is placed as a separate phylum under non-chordata." The reason is that hemichordates have only "a rudimentary structure in the collar region called stomochord, a structure similar to notochord." A stomochord is not a notochord; resemblance is not identity, so hemichordates remain non-chordates. NEET has used this exact distinction in match-the-column items pairing stomochord with Hemichordata.

Figure 2 The chordate body plan and the notochord dorsal hollow nerve cord notochord — mesodermal dorsal rod pharyngeal gill slits post-anal tail All four characters together define phylum Chordata; the notochord is the single naming criterion.

Figure 2. The chordate body plan after NCERT Figure 4.16. The notochord is a dorsal, mesodermally derived rod; above it runs the dorsal hollow nerve cord; the pharynx is perforated by gill slits; a post-anal tail extends beyond the anus.

Notochord in the three subphyla

Phylum Chordata is divided into three subphyla — Urochordata (or Tunicata), Cephalochordata and Vertebrata. The notochord is present in all three, but where and for how long differs, and that difference is one of NEET's most frequently asked points.

Where the notochord persists — by subphylum

NCERT Section 4.2.11
  1. Urochordata

    Larval tail only

    Notochord confined to the tail of the larva; lost in the adult.

    Ascidia, Salpa, Doliolum
  2. Cephalochordata

    Head to tail, lifelong

    Notochord extends from head to tail and persists throughout life.

    Branchiostoma (Amphioxus)
  3. Vertebrata

    Embryo only

    Notochord present during the embryonic period; later replaced by a vertebral column.

    All vertebrates

Urochordata and Cephalochordata are together called protochordates and are exclusively marine. In members of subphylum Vertebrata, NCERT states, "The notochord is replaced by a cartilaginous or bony vertebral column in the adult." This single sentence explains NCERT's famous conclusion: "all vertebrates are chordates but all chordates are not vertebrates." Every vertebrate has a notochord in the embryo, so every vertebrate is a chordate; but Urochordata and Cephalochordata are chordates that never develop a vertebral column, so the reverse statement fails.

One nuance NCERT records is worth noting for the trickier classes. In Chondrichthyes, the cartilaginous fishes, the notochord is "persistent throughout life" — a detail that distinguishes them within Vertebrata, where the notochord is otherwise an embryonic structure. NEET 2020 turned the general rule into a question by offering the false claim that in Urochordata the notochord extends from head to tail throughout life; that description actually belongs to Cephalochordata, so the statement was incorrect.

Animals belonging to phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits.

NCERT Class 11 Biology — Section 4.2.11

Pulling the two criteria together: segmentation and the notochord both peak in the most advanced phyla, but they are not the same kind of statement. Segmentation describes a repeated plan and is "present" in Annelida, Arthropoda and Chordata. The notochord describes a single dorsal rod and is the naming criterion of Chordata alone. Chordata is therefore the one phylum NCERT marks as having both features, while every non-chordate phylum lacks the notochord and most of them lack segmentation as well.

Worked examples

Worked example 1

Name the three phyla in which NCERT Table 4.2 marks segmentation as "Present", and identify which one is named after this very feature.

The three phyla are Annelida, Arthropoda and Chordata. Annelida is named after the feature: the word comes from the Latin annulus, meaning "little ring", because the body is "distinctly marked out into segments or metameres". The remaining eight phyla — Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Mollusca, Echinodermata and Hemichordata — are marked "Absent".

Worked example 2

A worm-like marine animal has a cylindrical body of proboscis, collar and trunk, and a rudimentary rod in the collar. Is it a chordate? Justify using the notochord criterion.

It is not a chordate. The animal is a hemichordate (e.g. Balanoglossus). The rudimentary collar rod is the stomochord, which NCERT describes only as "a structure similar to notochord" — it is not a true notochord. Since the defining criterion of Chordata is the formation of a genuine notochord, Hemichordata "is now placed as a separate phylum under non-chordata".

Worked example 3

"All vertebrates are chordates but all chordates are not vertebrates." Explain the statement using where the notochord persists.

Every vertebrate possesses a notochord during the embryonic period, so each one satisfies the chordate definition — hence all vertebrates are chordates. However, phylum Chordata also contains Urochordata (notochord only in the larval tail) and Cephalochordata (notochord from head to tail throughout life). These protochordates never form a vertebral column, so they are chordates but not vertebrates — hence not all chordates are vertebrates. In Vertebrata the notochord "is replaced by a cartilaginous or bony vertebral column in the adult".

Worked example 4

Select the animal groups that show all of: organ-system level of organisation, bilateral symmetry, and true coelom with body segmentation.

The correct set is Annelida, Arthropoda and Chordata (NEET 2019). All three are organ-system level, bilaterally symmetrical, true coelomates, and are the only phyla NCERT marks as segmented. Mollusca and Echinodermata are coelomates but their bodies are unsegmented, so any option pairing them with "segmentation" fails.

Common confusion & NEET traps

NEET PYQ Snapshot — Segmentation & Notochord

Real NEET previous-year questions on segmentation and the notochord, with year tags and worked solutions.

NEET 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.

  1. Annelida, Arthropoda and Chordata
  2. Annelida, Arthropoda and Mollusca
  3. Arthropoda, Mollusca and Chordata
  4. Annelida, Mollusca and Chordata
Answer: (1)

Why: Annelida, Arthropoda and Chordata are the only phyla that are organ-system level, bilaterally symmetrical, true coelomate and segmented. Mollusca is a coelomate but its body is unsegmented, which eliminates options (2), (3) and (4).

NEET 2022

Assertion (A): All vertebrates are chordates but all chordates are not vertebrates. Reason (R): Notochord is replaced by vertebral column in the adult vertebrates. Choose the most appropriate answer.

  1. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
  2. (A) is correct but (R) is not correct
  3. (A) is not correct but (R) is correct
  4. Both (A) and (R) are correct and (R) is the correct explanation of (A)
Answer: (4)

Why: Chordata has three subphyla — Urochordata, Cephalochordata and Vertebrata. In Vertebrata the notochord is replaced by a vertebral column in the adult, while protochordates never form one. So both statements are correct and the reason explains the assertion.

NEET 2020

Which of the following statements are true for the phylum Chordata? (a) In Urochordata notochord extends from head to tail and is present throughout life; (b) In Vertebrata notochord is present during the embryonic period only; (c) Central nervous system is dorsal and hollow; (d) Chordata is divided into 3 subphyla: Hemichordata, Tunicata and Cephalochordata.

  1. (c) and (a)
  2. (a) and (b)
  3. (b) and (c)
  4. (d) and (c)
Answer: (3)

Why: Statement (a) is false — head-to-tail lifelong notochord belongs to Cephalochordata, not Urochordata. Statement (d) is false — the three subphyla are Urochordata, Cephalochordata and Vertebrata, not Hemichordata. Statements (b) and (c) are correct.

NEET 2021

Match List-I with List-II: (a) Metamerism — (i) Coelenterata; (b) Canal System — (ii) Ctenophora; (c) Comb plates — (iii) Annelida; (d) Cnidoblasts — (iv) Porifera.

  1. (a)-(iv), (b)-(i), (c)-(ii), (d)-(iii)
  2. (a)-(iv), (b)-(iii), (c)-(i), (d)-(ii)
  3. (a)-(iii), (b)-(iv), (c)-(i), (d)-(ii)
  4. (a)-(iii), (b)-(iv), (c)-(ii), (d)-(i)
Answer: (4)

Why: Metamerism is the diagnostic feature of phylum Annelida, where the body shows external and internal segmentation with serial repetition of organs. Canal system belongs to Porifera, comb plates to Ctenophora, and cnidoblasts to Coelenterata.

FAQs — Segmentation & Notochord

Quick answers to the most common doubts on segmentation and the notochord as classification criteria.

What is metameric segmentation according to NCERT?

NCERT defines metameric segmentation as the condition in which the body is externally and internally divided into segments with a serial repetition of at least some organs. The earthworm is the textbook example, and the phenomenon itself is called metamerism.

In which phyla is true segmentation present?

In NCERT Table 4.2, segmentation is marked Present only in Annelida, Arthropoda and Chordata. It is marked Absent in Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Mollusca, Echinodermata and Hemichordata.

What is the notochord and from which germ layer does it form?

The notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development. Animals that form it are chordates; animals that never form it are non-chordates, namely Porifera to Echinodermata.

Is the stomochord of Hemichordata the same as a notochord?

No. Hemichordates have a rudimentary structure in the collar region called the stomochord, which is only similar to a notochord. Because they do not form a true notochord, Hemichordata is now placed as a separate phylum under non-chordata.

Why are all vertebrates chordates but all chordates not vertebrates?

Every vertebrate possesses a notochord at least during the embryonic period, so it qualifies as a chordate. However, the chordate phylum also contains Urochordata and Cephalochordata, which keep the notochord and never form a vertebral column, so not all chordates are vertebrates.

In which subphylum does the notochord persist throughout life?

In Cephalochordata the notochord extends from head to tail and persists throughout life. In Urochordata it is confined to the larval tail, and in Vertebrata it is present only in the embryo and is later replaced by a cartilaginous or bony vertebral column.

Related Topics
Animal Kingdom Back to the full chapter notes. Levels of Organisation Cellular to organ-system grades of body plan. Coelom Types Acoelomate, pseudocoelomate and coelomate plans. Phylum Annelida The metamerically segmented, true-coelomate phylum. Phylum Arthropoda Segmented bodies with jointed appendages. Phylum Chordata — Subphyla Urochordata, Cephalochordata and Vertebrata. Phylum Hemichordata Worm-like non-chordates with a stomochord.