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The word "animal" comes from the Latin word animale, neuter of animalis, and is derived from anima, meaning vital breath or soul. In everyday colloquial usage, the word usually refers to non-human animals. The biological definition of the word refers to all members of the Kingdom Animalia. Therefore, when the word "animal" is used in a biological context, humans are included.
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About Earthworms

Posted by WishbonE at 5:45 PM

Wednesday, July 2, 2008

About Earthworms / Nightcrawlers

Great naturalist Charles Darwin carefully studied Earthworms and wrote:

"...it may be doubted if there are any other animals which have played such an important part in the history of the world as these lowly organized creatures."

Earthworms (folk names are "dew-worm", "rainworm", "night crawler" and "angleworm") are very important animals that aerate the soil with their burrowing action and enrich the soil with their waste products (called castings). Good soil can have as many as as 1,000,000 (a million) worms per acre. Earthworms belong to a large phylum, the Annelida, or segmented worms. They belong to the Class Oligochaeta. This name means 'few bristles' and refers to the few bristles, or setae, on each body segment compared with the many setae of marine annelids in the Class Polychaeta ('many bristles'). There are four pairs of setae on each segment. These can be detected as a roughness if the animal is stroked from tail to head.
Earthworms are diverse enough to be broken into four major families, with approximately 3,000 known species. Earthworms range in size from several millimetres to two or three metres in length.

The brain, hearts, and breathing organs are located in the first few segments of the worm. It has five pairs of hearts! The rest of the inside of an earthworm is filled with the intestines, which digest its food. Earthworms eat soil and the organic material in it - like insect parts and bacteria. The mouth is covered by a flap (called the prostomium) which helps the earthworm sense light and vibrations. Tiny bristles (called setae) are on most segments of the earthworm's body. Earthworms breathe in the same way as their aquatic ancestors. They don't have lungs, but instead breathe through the skin. In order for gas exchange to take place this way, the outermost layers of an earthworm are thin and must be kept moist. Mucous is excreted onto the skin to keep it moist. It is also wet by body fluid which is excreted through 'dorsal pores' located along the dorsal (back) midline in the grooves between the segments. This need for moisture restricts their activities to a burrowing life in damp soil. They emerge only at night when the evaporating potential of the air is low, and retreat deep underground during hot, dry weather. Light-sensitive tissues near the worm's head enable it to detect light, so they can avoid venturing out by day.
Most earthworms are scavengers that feed on dead organic matter. They feed by passing soil through the gut, from which nourishment is extracted, or by eating organic debris, including leaves accumulated on the surface of the soil. This method of feeding does not require highly developed sense organs (such as eyes, which would be of little use underground) or food-catching structures, and earthworms never possess the often very remarkable and versatile head appendages developed in some of the free-swimming, carnivorous marine polychaete worms.

The digestive system is divided into a number of regions, each with a special function. Food that enters the mouth is swallowed by the action of the muscular pharynx, then passes through a narrow esophagus that has three swellings on each side. These are the calciferous glands that excrete calcium carbonate to dispose of excess calcium obtained in the food. The food then moves to the crop, which seems to serve only as a storage organ, and then to the muscular gizzard. With the aid of very tiny stones swallowed by the worm, the gizzard grinds the food thoroughly. Food is then digested by juices secreted by gland cells in the intestine. It is absorbed by blood vessels in the intestinal wall and from there distributed to the rest of the body. This is something we should appreciate because earthworm droppings -- called castings when deposited atop the ground -- are rich in nitrogen, calcium, magnesium, and phosphorus, and these are all important nutrients for healthy, prospering ecosystems. In your own backyard you might be able to confirm that grass around earthworm burrows grows taller and greener than grass just inches away. One study showed that each year on an acre (0.4 hectare) of average cultivated land, 16,000 pounds (7200 kg) of soil pass through earthworm guts and are deposited atop the soil -- 30,000 pounds (13,500 kg) in really wormy soil! Charles Darwin himself calculated that if all the worm excreta resulting from ten years of worm work on one acre of soil were spread over that acre, it would be two inches thick (5.08 cm).

In many soils, earthworms play a major role in converting large pieces of organic matter (e.g. dead leaves) into rich humus, and thus improving soil fertility. This is achieved by the worm's actions of pulling down below any organic matter deposited on the dried dirt, such as leaf fall or manure, either for food or when it needs to plug its burrow. Once in the burrow, the worm will shred the leaf and partially digest it, then mingle it with the earth by saturating it with intestinal secretions. Worm casts (see below) can contain 40% more humus than the top 6" of soil in which the worm is living.
As well as dead organic matter, the earthworm also ingests any other soil particles that are small enough—including stones up to 1/20 of an inch (1.25mm) across—into its gizzard wherein minute fragments of grit grind everything into a fine paste which is then digested in the stomach. When the worm excretes this in the form of casts which are deposited on the surface or deeper in the soil, minerals and plant nutrients are made available in an accessible form. Investigations in the US show that fresh earthworm casts are 5 times richer in available nitrogen, 7 times richer in available phosphates and 11 times richer in available potash than the surrounding upper 6 inches (150 mm) of soil. In conditions where there is plenty of available humus, the weight of casts produced may be greater than 4.5 kg (10 lb) per worm per year, in itself an indicator of why it pays the gardener or farmer to keep worm populations high.

Although each earthworm is hermaphroditic (having both male and female reproductive systems), it takes two worms to mate and reproduce. The reproductive organs are in the clitellum (the enlarged segments in the middle of an earthworm). The clitellum later forms a cocoon which protects the developing eggs.

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