Earthworm

Lore & Background

Earthworms exhibit an externally segmented tube-within-a-tube body plan with corresponding internal segmentations, usually having setae on all segments. They have a double fluid transport system: coelomic fluid within the coelom and a simple closed circulatory system, and respire via cutaneous respiration. As soft-bodied invertebrates, they lack a true skeleton but maintain structure through a fluid-filled coelom acting as a hydrostatic skeleton. Their central nervous system consists of two ganglia above the mouth connected to an axial nerve running the length of the body, with chemoreceptors concentrated near the mouth. Circumferential and longitudinal muscles in each segment enable movement, and similar muscles line the gut to propel digested food toward the anus.

Reader's Guide

Earthworms are significant as key detritivores and coprophages, breaking down organic matter and recycling nutrients in soil ecosystems. They serve directly as food for many low-level consumers, including wild boars, moles, blackbirds, centipedes, ground beetles, and flatworms. Their burrowing and feeding activities aerate soil and enhance its fertility. Most earthworms are hermaphrodites, exchanging sperm during mating, though many species are parthenogenetic. Their ability to eject coelomic fluid under stress, as seen in the Australian 'blue squirter earthworm,' demonstrates unique defensive adaptations. The taxonomic classification remains debated, with classical systems placing them in Opisthopora and cladistic studies suggesting suborder Lumbricina of Haplotaxida, reflecting ongoing scientific uncertainty.

Did You Know?

• Earthworms can range from 10 mm long and 1 mm wide to 3 m long and over 25 mm wide, depending on species.
• The longest confirmed earthworm is Amynthas mekongianus, found along the Mekong River in Southeast Asia, reaching up to 3 m.
• Earthworms have a central nervous system with a bilobed brain and a ventral nerve cord containing giant axons that transmit signals at speeds up to 32.2 m/s.
• Some earthworms, like the Australian Didymogaster sylvaticus, can squirt coelomic fluid as high as 30 cm when stressed.