Structural Organisation In Animals - Notes | Class 11 | Part 6: Earthworm - Morphology



Systematic position

Phylum     : Annelida

Class         : Oligochaeta

Genus       : Pheretima

Species      : posthuma

·   Earthworm is a reddish-brown terrestrial invertebrate that inhabits the upper layer of moist soil.

·   During day time, they live in burrows made by boring and swallowing the soil.

·   Common Indian earthworms: Pheretima and Lumbricus.


·   Earthworms have long segmented cylindrical body.

·   Number of segments (metameres): about 100-120.

·   Dorsal surface has a dark median mid dorsal line (dorsal blood vessel) along the longitudinal axis of the body.

·   First segment (peristomium or buccal segment) bears the mouth. A lobe called prostomium covers the mouth.

·   Prostomium is sensory in function and is used to force open cracks in the soil into which the earthworm may crawl.

·   In a mature worm, segments 14-16 are covered by a dark band of glandular tissue called clitellum.

·   Body has 3 regions: preclitellar, clitellar & postclitellar.

·   4 pairs of spermathecal apertures are found on ventro-lateral sides of intersegmental grooves (5th -9th segments).

·   A single female genital pore is present in the mid-ventral line of 14th segment.

·   A pair of male genital pores is present on the ventro - lateral sides of the 18th segment.

·   Many minute nephridiopores open on the body surface.

· All segments except the first, last and clitellum bear S-shaped setae, embedded in the epidermal pits. Setae can be extended or retracted. Their function is locomotion.


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