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If your textbook calls the earthworm "metamerically segmented", it means more than rings on the skin. The rings outside line up with walls and repeated organs inside. That inside-and-outside match is what separates true segmentation from animals that only look divided, such as the tapeworm.
What does metameric segmentation mean?
NCERT's Class 11 Biology chapter on the animal kingdom defines it in one line: in some animals, "the body is externally and internally divided into segments with a serial repetition of at least some organs" (NCERT, Animal Kingdom, section 4.1.5). The same paragraph names the earthworm as the example, calls this pattern metameric segmentation, and calls the phenomenon metamerism. Each repeated unit is a segment, also called a metamere.
A Dictionary of Zoology, quoted on Encyclopedia.com, says the same thing another way: "the repetition of organs and tissues at intervals along the body of an animal, thus dividing the body into a linear series of similar parts or segments (metameres)."
In an answer, the marks sit on three words: externally, internally and organs. Leave out the inside or the organs and you have described rings, not metamerism.
Which animals show true segmentation?
NCERT's Table 4.2 marks segmentation as present in three phyla only: Annelida, Arthropoda and Chordata (NCERT, Animal Kingdom). Every other phylum in that table, including Platyhelminthes, is marked absent, and the molluscan body is described as "unsegmented".
Annelida: NCERT calls them "triploblastic, metamerically segmented and coelomate", and the phylum name comes from the Latin annulus, little ring. Organs such as nephridia, nerves and muscles repeat serially and work in coordination (IAS Zoology notes on metamerism). For what each earthworm segment carries, see earthworm segments, setae and clitellum.
Arthropoda: segmented too, with a body of head, thorax and abdomen and jointed appendages. Our Annelida vs Arthropoda comparison sets the two side by side.
Chordata: the segments are less obvious from outside; muscle blocks (myotomes) and nerves are arranged segmentally (IAS Zoology).
Why is a tapeworm not truly segmented?
The tapeworm Taenia, listed by NCERT under Platyhelminthes, looks like a chain of segments called proglottids. A 2020 study in BMC Genomics describes how they form: strobilation "involves the repetitive generation of new proglottids in the base of the head (scolex), in the so-called neck region" (Paludo and colleagues, BMC Genomics). The same paper describes strobilation as the serial repetition of the worm's reproductive organs.
Two features make this pseudo-segmentation, also called pseudometamerism (IAS Zoology):
What repeats is mainly the reproductive system. Each mature proglottid contains "its own complete hermaphroditic reproductive system as well as its share of the common excretory and nervous systems" (Invertebrate Anatomy OnLine, Lander University). Nerve cords and excretory canals run the length of the worm, but each proglottid is built around its own set of sex organs. In an earthworm, each segment repeats nephridia, nerves and muscles as part of one body.
The order is reversed. In true metamerism, new segments are added just in front of the last part, the pygidium, so the youngest segments are at the back. In a tapeworm, the youngest proglottids are at the front, just behind the scolex.
How can you tell true segmentation from pseudo-segmentation?
Ask four questions. True metamerism needs a yes to the first three.
Do the outside rings match divisions inside the body?
Do at least some organs repeat in each unit?
Does each unit repeat the general body plan (such as nephridia, nerves and muscles), rather than mainly a set of reproductive organs?
Where are new units added: at the back, in front of the pygidium (true metamerism), or just behind the head, at the neck (tapeworm)?
Worked example. NCERT's exercise 13 asks: "Segmentation in the body is first observed in which of the following: (a) Platyhelminthes (b) Aschelminthes (c) Annelida (d) Arthropoda".
Platyhelminthes: the tapeworm passes question 2 but fails questions 3 and 4. NCERT marks segmentation absent. Cross it out.
Aschelminthes: roundworms have no segments. Cross it out.
Annelida and Arthropoda: both are segmented. NCERT describes Annelida before Arthropoda, and Annelida is the first phylum in Table 4.2 marked present. Answer: (c) Annelida.
The same option-by-option check works for animal symmetry types.
Did segmentation evolve once or many times?
Biologists still debate this. In a 2010 BioEssays paper, Ariel Chipman notes that molecular evidence has suggested a segmented common ancestor for all bilaterally symmetrical animals, while fossil and anatomical evidence argues against it (Chipman, BioEssays). He proposes that segmentation arose in parallel, by reusing older gene networks that control how the body axis forms and lengthens. It is not exam material, but it offers one explanation for why three distant phyla share the feature.
If terms like coelom or metamere still feel slippery, the Fermi Concept Library has a lesson for every concept in biology, with its place in the map. For practice with questions like the one above, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
