Q.Write down the salient features of Phylum Annelida. What is the role of Nephridia in these animals?

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 …

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.

…

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. …

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 …

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. …

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. …