The best known oligochaete is the Common earthworm Lumbricus terrestris (see above, and cocoons below) which can be up to 30 cm long, and is the largest earthworm in Northern Europe; however tropical earthworms can be much longer.
Lumbricus terrestris burrows deeper than most other European earthworms, and generally lives in a U-shaped burrow.
At night most of its body leaves the burrow to collect food - small bits of vegetable debris, leaves, twigs etc. Some of this is eats, and the rest it uses to plug up the burrow and eat later. When food is scarce it makes more extensive burrows, and can go as deep as 2 m. Most Lumbricidae don't go deeper than a few cm. And most do not collect food from the soil surface.
In L. terrestris cocoons (see the photographs above), more than one egg may be present at first, but usually only one worm develops. In other species many worms can develop in a single cocoon.
In captivity L. terrestris has lived as long as 6 years!
Earthworms head for the surface or upper layers of soil when it rains. When it is dry or cold they burrow deeper carrying shreds of leaves down with them. The earthworm burrows by simply eating whatever is in front of it. Its intestines absorb whatever is nutritious, and the rest is excreted.
Below is Octolasium cyaneum. This earthworm is commoner in wetter soils. I took this photograph on a day of torrential rain, and the quiet road was full of worms that had been flooded out and were making for drier ground.
Octolasium cyaneum's colour ranges from blue to purple to pink. It is redder at the front, and has a few yellow segments at the rear. Fully grown it is 8 - 14 cm long.
Below is Dendrobaena veneta, also known as Eisenia hortensis. It is commonly sold for composting and also as bait for fishing.
This worm tends not to come to the surface making it difficult to study, and giving it its other name - nature's plough. It is found in a small area in the Bass River Valley. Its head is dark purple, and it body a pinky-grey. Its egg capsule is the size, shape and colour of a cocktail sausage, and is laid approximately 20 cm deep in the soil.
Darwin spent over 40 years studying earthworms, and wrote a book about them and their earth-moving abilities (The formation of vegetable mould through the action of worms).
He found that in one acre the earthworms were bringing up 10 tons of worm casts a year, and an earthworm can digest its own weight in soil every 24 hours. In tropical soils it is thought that the amount is at least 20 times more as the worms are larger and are more active. This means that as much as 50% of the soil passes through the gut of earthworms each year, greatly accelerating the internalisation process of organic particles.
In the UK there can be over 7,000,000 earthworms per hectare of good pasture. This weighs more than the cattle grazing on the grass! But this is nothing compared to tropical forests where the biomass of earthworms can reach 50 tons per square kilometre.