The Earthworm is a terrestial worm of the family Vermes that inhabits the soils of temperate areas. The earthworms are annelids, meaning "rings", since they have a segmented body. Much as with crustaceans, the majority of annelids are aquatic or semi-aquatic creatures. The earthworm itself is still aquatic, although its ocean is only a millimetre of slime. Finding a niche on land was not easy for the earthworm, and as opposed to the fast, cheap and out of control arthropods, the earthworm holds a fragile hold on land. No matter how blasted, polluted or inhospitable a land environment is, it is bound to be swarming with insects of some type or another. On the other hand, it is quite possible to kill off all the earthworms on a piece of ground and make it impossible to return. It is unfortunately possible to do this without even trying.
The Earthworm is a rather simple animal. It has a digestive system, a muscular system, a reproductive system and a circulatory system. It even has a rudimentary nervous system, with perhaps the most primitive brain of any living creature. The only exterior feature of a worm's anatomy that is visible is the citellum, the pale swollen band towards the worm's head.
- Digestive system : The worms body consists of one big long tube of intestine, that moves the worms primary food, decaying plant matter along, taking nutrients out along the 100 or so segments that make up the worms body. The only real feature in the intestine is close to the head, where the worm has a gizzard, a sac full of very small pieces of sand, that help mash and grind the plant matter down so it can be easily digested. After that, it is just one segment of intestine after another until the worm's anus.
- The muscular system consists of two main types of muscles, the circular muscles that run around each segment, and longitudal muscles, that run down the length of the body. These muscles are very important, because tunneling through soil takes a lot of strength, especially considering the earthworms only way of moving is to expand and contract its body.
- The circulatory system consists of vessels taking nutrients from the gut, and delivering oxygen to the muscles. It does not have a true heart, but merely a muscular thickening of the vessles close to the head. One thing that the worm does have that is rather sophisticated is pigmented blood, meaning that oxygen travels attached to a metallic compound, instead of just through diffusion. This is a leap that most arthropods never made. Insects do not have true blood.
- The reproductive system is rather complicated, since earthworms produce both sperm and ova. In other words, they are hermaphrodites. There mating, which occurs whenever two worms happen upon each other in the soil, consists of a mutual exchange of sperm that fertilizes the other worms eggs. These are then secreted through the citellum.
- The nervous system The worm doesn't have much of a nervous system. Whether it has anything that could be called a central nervous system is debatable. A worm does not have any eyes, but it can detect light and vibration, and will pull away from them. It also has olfactory abilities to detect food.
Of course, the skin itself is an important organ in worms. Since the worm has no lungs or gills, all of the oxygen it takes in must come in through the skin through osmosis. For this reason, the skin must be kept wet at all times, since the oxygen exchange needs a wet membrane to move through. The worms skin is the closest thing to a lung it has.
The skin also houses the setea, four small hooks on each segment that the worm uses to dig itself into the ground and move forward.
That is the biology of the earthworm. Its ecology, of course, is just as important, as without the earthworm a great deal of plant matter would go undigested. The earthworm concentrates the nitrogen in plant matter into concentrated protein, that is then returned to the food chain through the work of worms many predators.
The role of the worm in aerating and turning over the soil is also well known.
That being said, there are a few myths that need to be cleared up. First, worms can not be cut in half, and grow into two new worms. If a worm is cut in half well below the citellum, it is possible for it to grow a new tail, but the tail section can not grow a new head.
The other myth is that worms come out of their burrows when it rains to avoid drowning. Worms are actually quite well suited for living under water (as long as it is well oxygenated), and can live in the water indefinitely. The reason that worms come up during the rain is so they can travel quickly overland to greener (or perhaps browner) pastures. The reason they stop and die on the sidewalks is because they can become parallized by the light and heat, and once they desicate, they are done for.