Q.Metameric segmentation is the main feature of:
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Phylum Annelida – The Segmented Worms
Imagine you are building a train out of identical carriages. Each carriage has its own set of wheels, its own floor, and its own walls, but they are all linked together to form one long, flexible body. That is the core idea behind annelids — the segmented worms.
Before annelids, most animals you have studied (like flatworms or roundworms) were either solid or had a simple fluid-filled cavity. Annelids are the first group in the animal kingdom to show true metameric segmentation: the body is divided into a series of repeating units called segments or metameres. Each segment contains its own set of muscles, nerves, blood vessels, and excretory organs. This is not just a surface groove — the internal partitions (septa) separate the segments internally as well.
Why does this matter? Segmentation allows for specialisation. Different segments can take on different jobs. In an earthworm, the front segments are specialised for feeding and sensing, while the rear segments handle waste. More importantly, segmentation allows the body to move efficiently — each segment can contract independently, creating a wave of motion that lets the worm burrow through soil or swim through water.
The Precise Statement
Phylum Annelida: Triploblastic, bilaterally symmetrical, coelomate animals with metameric segmentation (body divided into repeating segments), a closed circulatory system, and organ-system level of organisation. The body is covered by a thin, moist cuticle. Locomotion is aided by chitinous bristles called setae (or parapodia in some forms). Excretion is by nephridia. The nervous system consists of a dorsal brain and a ventral nerve cord with ganglia in each segment.
Key Features You Must Know
1. Metameric Segmentation – This is the defining feature. The body is divided into segments by internal septa. Each segment is a metamere. This is true segmentation (not just external rings), because the coelom is partitioned by septa, and each segment has its own nerve ganglion, blood vessel loop, and nephridium.
2. Coelom – Annelids are coelomate animals. The coelom is a true body cavity lined by mesoderm on both sides (peritoneum). It is filled with coelomic fluid, which acts as a hydrostatic skeleton — essential for burrowing and movement.
3. Closed Circulatory System – Blood flows entirely within blood vessels (arteries, veins, capillaries). There is a main dorsal vessel that pumps blood forward and a ventral vessel that carries it backward. In earthworms, five pairs of "hearts" (pseudohearts) connect the dorsal and ventral vessels. This is a major evolutionary advance over the open circulatory system seen in arthropods.
4. Excretory System – Each segment has a pair of nephridia (tubular structures) that filter waste from the coelomic fluid and release it outside. This is the first appearance of a dedicated excretory organ.
5. Nervous System – A dorsal cerebral ganglion (brain) connects to a double ventral nerve cord that runs the length of the body, with a ganglion in each segment. This is the "ladder-type" nervous system.
6. Locomotion – Setae (bristles made of chitin) project from each segment. In earthworms, setae anchor the body during peristaltic movement. In Nereis (a marine annelid), each segment bears parapodia — fleshy, paddle-like appendages with setae — used for swimming and crawling.
7. Reproduction – Most annelids are hermaphrodites (earthworm, leech) but cross-fertilise. Some (Nereis) have separate sexes. Development can be direct (no larval stage) or indirect (with a trochophore larva).
The Three Main Classes
| Class | Key Feature | Example | Habitat |
|---|---|---|---|
| Oligochaeta | Few setae per segment; no parapodia; clitellum present | Earthworm (Pheretima) | Terrestrial, freshwater |
One animal phylum is defined by a body plan repeated as a linear series of similar segments along its length. …
Metameric segmentation — division of the body into a linear series of similar repeating segments (metameres) — is the defining feature of phylum Annelida.
In animals such as the earthworm (Pheretima) and Nereis, the body is externally and internally divided into a series of ring-like segments, each of which (except the first and last) tends to repeat the same organs — nephridia, ganglia, blood vessels, muscles, and setae. This repetitive segmentation is called metamerism.
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- CBSE 2024Set ANNUAL1 markMCQQ.Segmentation in the body is first observed in which of the following ?(a) Platyhelminthes(b) Aschelminthes(c) Annelida(d) Arthropoda
›Reveal solutionSolution
Metameric segmentation first appears as a body-plan feature in phylum Annelida.
Body segmentation, where the body is externally and internally divided into segments with a serial repetition of at least some organ systems, is called metameric segmentation and the phenomenon is known as metamerism. Platyhelminthes (flatworms) and Aschelminthes (roundworms) show no true segmentation. Annelids (e.g., earthworm, Nereis, leech) are the first group in the animal kingdom to show true met …
- CBSE 2024Set ANNUAL1 markMCQQ.Metameric segmentation is the main feature of:(a) Arthropoda(b) Annelida(c) Coelenterata(d) Echinodermata
›Reveal solutionSolution
Metameric segmentation — division of the body into a linear series of similar repeating segments (metameres) — is the defining feature of phylum Annelida.
In animals such as the earthworm (Pheretima) and Nereis, the body is externally and internally divided into a series of ring-like segments, each of which (except the first and last) tends to repeat the same organs — nephridia, ganglia, blood vessels, muscles, and setae. This repetitive segmentation is called metamerism.
…
- CBSE 2024Set zoology-sz1 markMCQQ.Nephridia help in excretion and osmoregulation in ___.(a) Arthropoda(b) Annelida(c) Aschelminthes(d) Mollusca
›Reveal solutionSolution
Nephridia are the excretory/osmoregulatory organs unique to annelids (e.g., earthworm), so the answer is (b) Annelida.
Step 1 — What is a nephridium?
A nephridium is a tubular, ciliated excretory structure found in annelids. It opens internally into the coelom through a ciliated funnel called the nephrostome, and opens externally through a nephridiopore.
Step 2 — Function.
Nephridia perform two related jobs:
- Excretion: they remove nitrogenous wastes (mainly ammonia and urea) from the coelomic fluid and blood.
- Osmoregulation: as fluid passes along the nephridial tubule, useful salts and water are selectively reabsorbed back into the blood capillaries surrounding it, while excess water and wastes are voided — this keeps the body fluid concentration stable.
Step 3 — Where they occur. …
- CBSE 2021Set ANNUAL1 markMCQQ.In which phylum closed circulatory system is present?(a) Annelida(b) Arthropoda(c) Mollusca(d) Hemichordata
›Reveal solutionSolution
Among the options, only Annelida (e.g. earthworm) circulates blood entirely within vessels.
In a closed circulatory system, blood flows through a closed network of blood vessels and never directly bathes the tissues. Annelids such as earthworm and leech have this system, with blood confined to vessels and a few contractile 'hearts'. Arthropods and Molluscs, by contrast, have an open circulatory system where blood …
- CBSE 2020Set ANNUAL1 markMCQQ.Choose the wrong statement among the following:(a) In earthworm, two pair of male genital pores are present.(b) Setae help in locomotion of earthworms.(c) The body wall of earthworm consists of Circular muscles and longitudinal muscles.(d) Typhlosole is part of the intestine of earthworm.
›Reveal solutionSolution
Statement (a) is wrong: Pheretima (the common earthworm) has only one pair of male genital pores, on segment 18, not two pairs.
Checking each statement against earthworm (Pheretima) anatomy:
- (a) "Two pair of male genital pores are present" — INCORRECT. Earthworms have only ONE pair of male genital pores, situated mid-ventrally on the 18th segment, through which sperm are released during copulation.
- (b) "Setae help in locomotion" — CORRECT. Chitinous setae (chaetae) project from the body wall in each segment (except the first, last, and clitellar segments) and grip the substratum, assisting in locomotion by anchoring parts of the body while other parts extend or contract.
- (c) "The body wall of earthworm consists of Circular muscles and longitudinal muscles" — CORRECT. The body wall has an outer layer of circular muscles and an inner layer of longitudinal muscles, whose alternate contraction produces peristaltic, worm-like locomotion. …
- CBSE 2019Set ANNUAL1 markMCQQ.The animals of which of the following have closed circulatory system.(a) Annelida(b) Arthropoda(c) Mollusca(d) None of these
›Reveal solutionSolution
Among the four options, only Annelida has a true closed circulatory system; Arthropoda and Mollusca (except cephalopods) have an open (lacunar) system.
In a closed circulatory system, blood is confined within blood vessels (arteries, veins, capillaries) and flows under regulated pressure — this allows precise distribution of blood to organs.
In an open circulatory system, blood (haemolymph) is pumped out of the heart into open spaces called sinuses/haemocoel, where it directly bathes the tissues before slowly draining back to the heart.
- Annelida (e.g., earthworm, leech): closed circulatory system — blood moves through dorsal and ventral vessels connected by lateral hearts. …
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