Oligochaeta - Class of Annelida, containing the earthworms such as Lumbricus and a number of freshwater forms. The following account is of Lumbricus terrestris but the main features are the same in the whole class. There are about 150 segments-all basically alike. The most noticeable external feature is the clitellum or 'saddle' which is a thicker region between segments 32 and 37 arid which is concerned with reproduction. Each segment other than the first and last has four pairs of small chaetae that assist movement and are responsible for the roughness of the worm.

The movement of a worm can be studied by allowing it to crawl over a piece of rough paper. The bristles at the front end are withdrawn and the front end is pushed forward. Circular muscles in the body wall contract and the pressure on the body fluids causes the body to lengthen. Then the bristles are extended again and grip the surface. Bristles at the hind end are withdrawn and, by contraction of the longitudinal muscles, the hind region is drawn up towards the front. The bristles are again extended to anchor the hind end and the front moves forward once more.

This type of movement is rather slow but the worm can also move very fast. If it is disturbed when partly out of its burrow it can with draw very fast. If it is disturbed when partly out of its burrow it can withdraw very rapidly. Some large nerve fibres running the whole length of the body carry messages rapidly to all the muscles and they contract quickly. As the hind end of the worm is fixed in the burrow by means of the bristles, the result is that the whole body shortens and retreats into the hole. The worm makes use of the two muscle layers in burrowing too. The circular muscles contract, making the body thin. It can then squeeze into cracks between soil particles. Then the longitudinal muscles contract and the pressure on the body fluids makes the worm fatter, thus entarging the hole and allowing the worm to pass in.

Internally, the most noticeable feature is the food canal extending the whole length of the body. It is generally surrounded by a mass of yellow chloragogen cells that are concerned with excretion. The mouth opening (in segment one) leads into the small buccal cavity. In segment three this passes into the muscular pharynx which occupies the next three segments. Glands in the pharynx secrete mucus and enzymes that mix with the food.

On the front of the pharynx there are two small masses of nerve tissue connected by a 'collar' of nerves (commissures) to the main nerve cord running underneath the gut. These masses of tissue form the brain of the worm. Behind the pharynx comes the oesophagus a narrower part of the gut which is surrounded by five pairs of blood vesscls called. pseudohearts. These contract rhythmically and carry blood from the dorsal vessel to the vessel underneath the gut. Bulging out from the oesophagus are a number of small pouches which secrete calcium carbonate. In this way the worm gets rid of unwanted calcium and carbon dioxide.

Worms feed by swallowing soil as they go through the earth. Much of their tunnelling is done by swallowing the soil as well as by forcing the particles aside. The soil goes through the oesophagus and into the soft walled crop which probably secretes digestive juices. Behind the crop is the gizzard where the soil particles are crushed to release all digestible material (i.e. decaying plant and animal remains). From the gizzard it all passes to 'the intestine where the food is further digested and absorbed. The undigested soil particles, together with a fair amount of calcium carbonate pas out of the worm as worm casts. Most worms, including Lumbricus, deposit their casts under the surface but a few species make surface casts. This ploughing action of worms is of great importance and with out it, soil would soon become compacted and sterile.

When disturbed, worm quickly begin to burrow again for although they have no eyes, they do not like the light. They often come to the surface at night and drag leaves or other material into their burrows. They return to their burrows when it gets light, hence the expression the early bird gets the worm. Worms rarely come to the surface in dry weather or when it is very cold.

Earthworms are hermaphrodite .animals. When the worm is opened, the large white lobes of the seminal vesicles are clearly seen around the oesophagus in segment'; 9 to 13. This is where the male cells are stored while they mature. To examine the reproductive system fully, the food canal must be cut away carefully. In segments 9 and 10 there are two pairs of spermathecae and in segment 13 a pair of tiny ovaries where the eggs are produced.

If the mid part of the seminal vesicles is opened, it may be possible to see the tiny testes where the male cells are actually produced and the funnels which lead to the outside. These funnels .open to the outside in segment 15 while the funnels carrying the eggs open in segment 14.

Although both male and female structures are present in each worm, two individuals must come together to reproduce. This usually happens on damp nights when the worms leave their burrows. Two worms come together, each with segments 9 to 11 opposite the saddle of the other, They secrete a lot of mucus around themselves and then male cells are exchanged.

Sperms leave the opening on segment 15 and run into a groove that extends backwards to the saddle. A series of muscle ripples carries the sperms along to the saddle from where they pass into the openings of the spermathecae of the other worm. The two worms then separate.

In each worm the saddle then produces a cocoon made up of slime cells. A dozen or so eggs pass back alon a groove into the cocoon which then becomes detached tram the body and slides off towards the front. On the way it picks up sperms pushed out of the spermathecase (i.e. sperms from the other worm). These sperms fertilise the eggs. The brownish cocoon is shed into the soil and is about the size of a pea. Usually only one embryo survives in each Lumbricus cocoon but several cocoons are produced by each adult until all the sperms have been discharged from the spermathecase.

After a few weeks the young worms leave the cocoons and those that escape the attention of moles birds and other enemics may live for two or three years. Unwanted carbon dioxide produced in respiration is got rid of by combining it with excess calcium absorbed from the soil to form calcium carbonate (CaC0₃). This then passes out with the undigested soil. Waste nitrogenous material is excreted via the yellow cells surrounding the gut.' These cells also store fats. The yellow cells secrete the nitrogen containing substances into the body fluid.

This then passes into the nephridia paired tubes that occur in all but the first three and the last segments. The inner end of the nephridium is the nephrostome. It is a tiny funnel covered with cilia which beat continuously and help to move the fluid down into the tube. If a nephrostome is removed from a freshly killed worm and looked at under the microscope it may even be possible to see the boating of the cilia. The tubes open to the outside by tiny pores. Blood vessels are closely connected with the coiled tubes and as the fluid moves, down towards the pore, useful salts are reabsorbed. Excess water and the unwanted materials then pass out of the body.

Ommatidium - Unit of compound eye.

Onychophora - Primitive class of Arthropoda which, in the possession of cilia and some other features, resemble annelid worms. The class contains Peripatus and realted animals which are caterpillar like, having numerous stumpy but unjointed legs. They are found in most of the warmer parts of the worls where they live under logs and in other damp places.

Operculum - The cover protecting the gills slits of bony fishes. Also the horny plate with which many gasteropods close their shell when at rest.

Oral - Concerning the mouth.

Orbit - Bony cavity housing the eyd of vertebrates.

Order - Category used in classification.

Ordovician Period - Period of geological time. Began about 420 million years ago.

Organelle - Part of a cell, especially of protozoan, forming a distinct unit. E.g. a flagellum.