The oligochaeta are mainly terrestrial or freshwater animals. There are over 3,100 species worldwide and 26 native species in Great Britain. Strangely, there are no native earthworms in the upper part of North America as they all went extinct in the last ice age. All the earthworms found in this region are introduced, and are thought to be having an adverse effect on forests by eating small seeds.
Earthworms range in length from less than 1 mm long to 4 m long, and 2 cm in diameter in the case of the giant Gippsland earthworm (see below).
Many people find it hard to believe that they have hairs or chaetae, but it is the chaetae anchoring it in its burrow that makes it so hard for birds to pull an earthworm up.
If you put an earthworm on a piece of paper you can hear the scratches of the chaetae as the worm moves. Do remember to handle the worm gently and put it back where you found it after it has entertained and educated you.
Respiration, breathing. Oligochaetes have no respiratory organs. Gas exchange (breathing) takes place over the entire surface of the body. So if the worm's skin becomes dry it starts to suffocate.As in polychaetes each segment has muscles, chaetae, excretory organs and nerve branches. Mature Oligochaetes have a clitellum which is always located closer to the head end; this is an unsegmented band or saddle located about a third of the way back from the head, and can be seen in the drawing above.
They closed circulatory system is a network of blood vessels with oxygen-carrying haemoglobin. In each segment there is a pair of vessels linking the dorsal and ventral vessel that run along the entire length of the body.
The five pairs of vessels around the esophagus function as hearts to pump blood through the system. In each segment there is a pair of metanephridia leading to pores through which metabolic wastes are discharged.
The skin has many tiny blood vessels to gather oxygen across the moist film covering it.
Below is a diagram of the nervous system of Lumbricus terrestris.
To dissect an earthworm pin it with the dorsal side facing up and cut from the head to the clitellum.
Most oligochaetes have no eyes, but there are a few aquatic species that have simple eyes. However ologochaetes do have photoreceptor cells in their skin, so they can sense changes in light levels.
Most are negatively phototactic to bright light. This means that they will automatically move away from any bright light source.
Most are scavengers, feeding on dead organic matter, especially vegetation, and along with their food they swallow large quantities of soil. The worm digestive tract, ventral nerve and longitudinal blood vessels penetrate the septa (see left, below and above). The digestive tract, from the mouth end, has a series of specialised regions.
Mating (see the drawing below) usually takes place at night in late summer, but they will mate in spring and autumn if conditions are favourable.
The mature worm emerges from its burrow and when it encounters another worm of the same species and reproductive state they lie side-by-side, head to head. The head ends become enveloped in a mucous tube which holds them together and enables sperm exchange.
Most earthworms hermaphrodite, and during copulation each worm will pass spermatheca to the other's clitellum which secretes the mucous holding both worms together. Mating can last for four hours.
A few days after mating the clitellum secretes a substance that encircles it. This becomes the cocoon wall. Into the space between the clitellum and the cocoon wall the clitellum glands secrete albumin. Next the cocoon moves forwards towards the head end of the worm. Eggs and sperm are released into the cocoon.
As it passes off the head the ends seal, see the earthworm cocoons above and below.
In terrestrial species the cocoon is left in the soil, and often resembles a large brown/beige seed. See left and right which shows the same cocoon, with that on the right photographed at a later date to that on the left, the scale on the left is in mm.
Above is a series of drawings which from left to right showing the gradual movement forward of an earthworm.
The muscles in the body wall do not have anything firm to attach to. However as the body is fluid-filled and has a limited volume the muscles can deform either the whole body, or segments of the body by contracting the circular and longitudinal muscles alternately.
In the earthworm each segment is an independent compartment. That is why it is still able to move off even when cut in half.
The best known oligochaete is the Common earthworm Lumbricus terrestris (see top, and cocoons above) 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.