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.

Mating. 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 are hermaphrodite, and during copulation each worm will pass spemathacae to the other's clitellum which secretes the mucous holding both worms together. 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 sectrete 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. In terrestrial species the cocoon is left in the soils, and often resembles a large brown/beige seed See below 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. In aquatic species it is left on the bottom amongst mud and debris, or attached to vegetation. Tubifex (see below) cocoons are 1.6 x 0.8 mm and the common earthworm's are 7 x 5 mm, both are oval. When conditions are good a cocoon can be formed every 3 or 4 days.

Feeding. Most are scavengers, feeding on dead organic matter, especially vegetation. The worm digestive tract, ventral nerve and longitudinal blood vessels penetrate the septa (see left and above which shows the earthworm). The digestive tract, from the mouth end, has a series of specialised regions. The pharynx contracts to help pump food into the mouth. The glands in the pharynx produce mucous and saliva. The pharynx open into the esophagus, and this opens into the crop which is a storage chamber for food particles before they pass into the muscular gizzard. In the gizzard the food is ground up before it is passed into the intestine. The intestine is where food is digested.

The 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. To dissect an earthworm pin it with the dorsal side facing up and cut from the head to the clitellum. The skin has many tiny blood vessels to gather oxygen across the moist film covering it.

Eyes. Most oligochaetes have no eyes, but there are a few aquatic species that have simple eyes. However ologochaetes do have photoraceptor 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.

Earthworms, also known as night crawlers and they have no eyes. Strangely, there are no native earthworms in the upper part of North America. All the earthworms found in this region are introduced. The best known oligochaete is the earthworm Lumbricus terrestris (see above left) which can be up to 30 cm long; however tropical earthworms can be much longer.

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.

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 mineralization 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.

The giant Australian Gippsland earthworm 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.

Easy experiment to see soil mixing by earthworms.

Materials
1 litre glass jar or similar clear-sided container
400 ml sand
400 ml garden soil
leaves
5 earthworms
dark paper or cloth to go around sides of jar

Method
1. Moisten the soil and the sand
2. Place the sand in the bottom of the container and flatten
3. Place the moistened garden soil on top of the sand
4. Place a few leaves on the surface
5. Place the worms on top
6. Put the covering or cloth around the sides of the jar

If you have a lid make sure there are perforations to let in air. Moisten the surface of the siol to prevent it drying out. Lift the cover from the sides and you will see how the progress of the worms tunneling mixes the soil with the sand until both are completely mixed.

Class Hirudinea

These are the leeches, and are considered the most advanced annelid class . They differ from the other two Classes in two important features. The segments at either end have been modified to form attachment suckers around the mouth and another (usually much smaller) at the posterior end. The sucker around the mouth is isially the smaller. Their body functions as a single hydrostatic skeleton as the internal divisions, or septa, have been lost. Also they do not have Chaetae. Leeches are predominantly freshwater animals, but some are marine and a few terrestrial. They range in size from 1 - 30 cm in length, but most are between 2 - 5cm long. The longest is the Giant Amazonian leech, Haementeria ghiliana. Like the oligochaetes they are hermaphroditic, but their clitellum only appears during the breeding season. Gas exchange occurs through the skin, except for some of the fish leeches with gills. They can have a number of tiny black eyes.

Leeches have a fixed number of segments, this is unlike the other Annelid Classes; the number of segments varies with species but is most commonly 17, 31 or 34. They appear to have many more segments because externally each has superficial grooves. Leeches have a looping movement, but are also very good and graceful, swimmers, they flatten their body into a ribbon-like shape and propel themselves forward in a series of vertical undulations.

Most leeches are fluid feeders. Blood-sucking leeches drink as much blood as they can from their victims. This blood is stored in special sacs at the side of their intestine and digested later. Most can consume 3 times theri own body weight in one meal. This allows the leech to survive long periods without food. They have two brains - one at the head end and the other at the tail end.

Freshwater leeches attach to their hosts only long enough to have one blood meal.

Hirudo medicinalis (the medicinal leech) is the biggest freshwater leech in the UK and is 6 - 8 cm long, but lengths of 20 cm have been recorded. It used to be fairly widespread but once bridges replaced fords and drinking troughs replaced ponds it became very difficult to find in the wild as it no longer had easy access to its victims. It has strong teeth which make a Y-shaped cut in the skin, and an anticoagulant in its digestive juices stops the clotting of the victim's blood. It cam consume 2 - 5 times its own body weight in blood, and go without feeding for 18 months. A full meal can take around 200 days to digest. Medicinal leeches are raised commercially to obtain hirudin - the anti-coagulant used to prevent blood clots. It has 10 eyes arranged in 2 rows. It is dark olive with dull reddish-black lines down its sides.

The horse leech (Haemopis sanguisuga) is mis-named as it feeds off worms, insects and dead animals. Also unlike other leeches it swallows its prey whole. It is about 5 - 6 cm long, and has 10 eyes arranged in 2 rows. It is dark green or brown with a pale stripe down the sides.

Placoblella sp. (far left) feeds mainly on turtles, but will also feed on alligators if there are no turtles.

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