Phylum Annelida Oligochaeta
The Oligochaetes are well-segmented Annelids, most with a spacious coelom that is used as a hydroskeleton. The name means "few-bristled". Their setae or "bristles" are generally few in number and they lack the parapodia of the polychaeta. They have external fertilization, but copulate and store sperm in a receptacle called a spermatheca. Like the leeches, they have a clitellum which secretes a "cocoon" into which both eggs and sperm are deposited and which acts as an incubator for the embryonic worms. They lack a trochophore larval stage.
This taxon contains mainly freshwater and semi-terrestrial forms, including the earthworms, the tubificids, pot worms and ice worms, blackworms and many interstitial marine worms. Most are detritus feeders. Some genera are predaceous.
One often sees earthworms come to the surface in large numbers after a rainstorm. There are four theories for this behavior.
The first is that the waterlogged soil has insufficient oxygen for the worms; therefore, earthworms come to the surface to get the oxygen they need and breathe more easily. However, earthworms can survive underwater for several weeks if there is oxygen in it, so this theory is rejected by some.
Secondly, some species come to the surface to mate. This behavior is, however, limited to a few species.
Thirdly, the worms may be using the moist conditions on the surface to travel more quickly than they can underground, thus colonizing new areas more quickly. Since the relative humidity is higher during and after rain, they do not become dehydrated. This is a dangerous activity in the daytime, since earthworms die quickly when exposed to direct sunlight with its strong UV content, and are more vulnerable to predators such as birds.
The fourth theory is that as there are many other organisms in the ground as well and they respirate as any animal does; the carbon dioxide produced dissolves into the rainwater; it forms carbonic acid and the soil becomes too acidic for the worms and they come seek neutral nourishment on the surface.
The subclass Oligochaeta contains all the animals commonly thought of as 'earthworms'. Long thin worms with no obvious appendages to their bodies and greatly reduced heads so that when the animal is still it is not sometimes obvious which end is the head and which is the tail. As a group they are all morphologically similar and taxonomic division is often made far more on the basis of internal characteristics, particularly the positioning of the genitalia.
The Oligochaeta are the second largest group of the Annelida, with 3,100 known species they make up about one third of the phylum. Within this diversity of species there are aquatic forms, both freshwater and marine and also many terrestrial species. In terms of diet, the smaller species are often predatory while the larger species are soil or mud feeders. There are also a few parasitic species.
The 'Oligo' in Oligochaeta means 'few' just as the 'Poly' in Polychaeta means many, thus the Oligochaeta are the animals with few chaetae, or few bristles. Normally, as in the common earthworms, Oligochaetes have 8 small chaetae per body segment. These are usually arranged in four groups of two around the body. The Oligochaetes are also distinguished from the Polychaeta in that they are hermaphroditic and in that they possess a clitellum as adults. This is an organ situated on the anterior section of the animal that is important in reproduction.
What the Oligochaeta lack in terms of species number, in comparison with the Polychaeta, they make up for in numbers of individuals. In some places in Britain there are 1 ton common earthworms per acre which move well over 10 tons of soil per year. Common earthworms are quite large, in comparison with many soil animals and they total numbers present in a given soil vary enormously in different soil types, habitats and soil usage regimes. Generally speaking deciduous woodland and orchard soils support the highest numbers, also temperate soils tend to support higher number than tropical soils. Over all soil types the range per square meter of soil is from less than 1 to more than 800 individuals. Other groups of Oligochaetes are smaller and can be packed more densely into a similar area, thus in the mud of the River Thames you can find more than 40,000 Oligochaetes per meter square, mostly if not entirely Tubificids, while further inland it is not difficult to find up to 300,000 or more Enchytreids per meter square of moorland soil.
Earthworms are enormously important in the construction, fertility maintenance of the soil and were described by Aristotle as 'the intestines of the earth'. Numerous soil scientists have been equally fascinated by the amount of work done by them, Charles Darwin said of them "It may be doubted whether there many other animals which have played so important a part in the history of the world as these lowly organized creatures".
Earthworms are miniature topsoil factories, earthworms make soil, all other living things eventually pass through an earthworm on the way to becoming soil, and nearly every atom in your body has been in an earthworms stomach before it was part of you, (salt, and some water are the an exceptions). Earthworm castings are rich in all the minerals necessary for plant growth in a water soluble form so that they are immediately available for plant use. All the soil you have ever seen has passed through the stomachs of numerous earthworms to become what it is. The best way to deal with those unwanted earthworm casts on your lawn is to collect them and use them as potting compost, no manufacturer can make anything better suited to the growth of seedlings.
It has been scientifically proven that earthworms increase the productivity of many soils, in some cases doubling or tripling crop yields. They do this by improving the structure of the soil, by bring nutrients up to the surface layers of the soil from deeper down and assisting in the break down of organic matter in, or on the surface of the soil.
This has resulted in a huge market in worms springing up in the second half of the 20th century in Western Europe and USA. In many cases this has been good, but in others, ignorance has resulted in the efforts and money spent being wasted. Soils have a definite worm carrying capacity which relates directly to the amount of organic matter in, and regularly added to, them (yearly in natural environments). Adding more worms than the soil can carry, or adding worms to soils poor or deficient in organic matter will result in only a small temporary increase in fertility resulting from the nutrients released from the dead worms and not from the work they have done. Further more not all worms are suitable for all soils and not all worms are active in soil creation, in particular Eisenia foetida, a commonly sold species which is very useful in composts and dung recycling is useless in, and will not survive in, ordinary field soils.