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The Earthworm

Worm n.b.: external features:

position of:
saddle or clitellum

elongated cylindrical body shape - streamlined, with no sideways projections to hinder movement through soil

body (length up to 25 cm) divided into segments (100+ ?) - each built on a similar plan - each has 4 pairs of chaetae or "bristles" - some internal duplication e.g. 5 pairs "false hearts"

skin is moist - coated with a thin layer of mucus - important for respiration - but no mechanism for reducing water loss.

Feeding ( "detritus feeding")

Earthworms feed by ingesting (taking in and swallowing) soil which is rich in organic matter, but dead vegetable material may be collected at night from the soil surface and later consumed. Earthworms do not feed on live plant material.

Food is broken down mechanically by grinding in a tough gizzard - thus even coarse gritty particles are reduced into finer ones. The organic matter (carbohydrates, fats and proteins) is broken down chemically by enzymes and converted into simpler soluble substances (digested) and the worm then absorbs these in solution into its blood.

Indigestible material - mainly soil particles of mineral origin - pass out at the anus, together with the faeces - egestion. Some species deposit this at at the soil surface as "worm casts".

Nitrogenous compounds from the breakdown of proteins are excreted and pass into the soil. Practically every segment has 2 primitive kidneys called nephridia which carry out excretion and osmoregulation.

Role in ecosystem

As a scavenger - collecting partially decomposed material (fallen leaves etc) and dragging it to its burrow - recycling plant material and moving it into the soil where other organisms can decompose it.

There are large numbers of earthworms in land of good fertility, but not in waterlogged or poor sandy soils.

Eaten by birds, e.g. blackbirds, thrushes, robins
and mammals, e.g. hedgehog, badger, fox, mole etc.

Effects of earthworms on soil

Beneficial :

Aeration and Drainage
Burrows in the soil formed by pushing loose soil aside, or swallowing more compact soil, act as passageways for the entry of air and rainwater into the soil. Plant roots need air to respire. Depth of burrows is usually up to 50 cm, but may be deeper - up to 2.5m. In cold and dry weather, earthworms are deeper down.

Addition of Humus
Organic matter dragged into soil decays gradually under the action of bacteria and fungi in moist conditions to become humus.

Soil taken in at one place becomes deposited elsewhere, causing a gradual turnover in the soil. This is especially important in breaking down the distinctions ( "horizons") between different layers in the soil. Worms bring mainly fine soil particles upwards as well as grinding it in the "gizzard" to produce a good "tilth", partly neutralising acid soils, and taking organic matter from the surface downwards.

Harmful :

Worm casts on the surface (produced by only a few species) are unwelcome on lawns & sports pitches - especially bowling greens etc. Some autumn lawn dressings contain worm-killing chemicals.
Moles feed on worms - so one way of discouraging moles is to water ground with such a worm-killing chemical.

No eyes, but front end (anterior) is more sensitive - to touch, vibrations, light, humidity, temperature, chemicals
2 sets of muscles in each segment contract alternately in waves: circular make segments long & thin - extension; longitudinal make segment short & fat - anchorage (with chaetae).
Hermaphrodite - both male & female organs in same body, but cross-fertilisation occurs. Mating involves exchange of sperms, then eggs are fertilised by partner's sperms, stored in spermathecae, as they are laid in a cocoon. Only one worm hatches in 12 weeks.
Oxygen dissolves in mucus film then diffuses through epidermis to blood capillaries containing haemoglobin ( upper surface darker red)
Carbon dioxide diffuses out in opposite direction. No "breathing".
Hydrostatic - liquid in space between body wall & gut.
Phylum: Annelida
Class: Oligochaeta
Genus: Lumbricus
Species: terrestris

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