Segmented Worms Facts

Segmented worms, such as the earthworm, the sandworm, and the leech, are divided into sections.

There are 9,000 species of segmented worms. These

are worms that have bodies divide into sections. The segmented worms, like the earthworm, are more advanced in their construction than flatworms as they possess a complete digestive tract with an anus to allow undigested matter to pass from the body. They also have a circulatory system including hearts that pump the blood through the system and also have an excretory system to rid the body of toxic products of respiration. I will define segmented worms in this article to hopefully increase your knowledge of these interesting species of worms.

The earthworm is a segmented worm that has a body wall consisting of an outer skin and two layers of muscles. Around the outer muscle layer are fibers that go around the body, the worm will become thin and long when these muscle layers contract. There are inner muscles running lengthwise that when contracted will make the worm become short and thick. Each segment has a flexible outer wall (a cuticle) surrounding its own coelomic chamber. The repeated chambers form a long, large body. For sensory perception, and for getting food, some segments especially those near the head, are modified. The flexible segments have a set of paired antagonistic muscles and a set of outward-projecting bristles (setae). There will be contractions and expansions of circular and longitudinal muscles passing through the segmented body. With these waves, bristles on contracted segments will grip the ground, keeping the body from slipping back when the other segments expand. The earthworm is able to move forward with a lot of coordination. Thus in the earthworm we see a form of "walking."

Earthworms have a tubular system that is linked with the circulatory system. Except for the first three head units and last tail unit, each body segment contains a pair of water-regulating tubules called nephridia. These tubes are the excretory system of the earthworm. At the beginning of each of these tubules (nephridia) a fluid-draining funnel leads away from the segment just ahead. From the funnel end, the tubule loops up and around a capillary bed, enlarges into a storage bladder, then ends in its own pore to the outside. When bodily fluids pass through this tubule system, solute and water levels are controlled. The water is then conserved in the storage bladder. When the body salts drop, they follow their concentration gradients and move out of the tubules, into the interstitial fluid, then back to blood capillaries. Salts in the tubules can be reabsorbed by the body as they don't automatically leave with outgoing water.

It is also possible to see a greater level of complexity in the nervous system. If there were not enough control present over the activities of the longitudinal, oblique, and circular muscles then there would be an anarchy of the body segments. This would certainly mean the worm could not move. There is a double nerve cord that travels the length of the body of the earthworm. Body movements are made because of this cord. The cords are swollen with cells controlling local activity and the brain integrates the sensory input.

There are blood vessels that move red blood around the body. Ten hearts help to keep the blood moving. Peristaltic contractions of smooth muscles keep blood moving through the system. The hearts are just short sections of blood vessels that contract and force the blood along the body. Dissolved from the intestine is carried by the blood to all parts of the body. Carbon dioxide from cells to the skin and oxygen from the outer to the inner skin is carried by the blood.

The earthworm has a body cavity through which the intestine passes with a front part that is modified to form special structures. The muscular throat sucks food into the food storage organ and it has a gizzard (a muscular organ in which food is ground into smaller pieces) that will grind food. They have an intestine (a region where chemical digestion and absorption occur). The intestine is tube-shaped and runs the entire length of the worm. It has a very tiny brain that is connected to a nerve cord that lies under the intestine. This comprises the nervous system of the earthworm.



The earthworm has a complete digestive system consisting of a tube with an opening at one end for taking in food, and an opening at the other for eliminating undigested residues. In between these two openings, food moves in one direction through regions specialized for processing and for transport. The earthworm does show discontinuous feeding patterns as its food may not be available at all times. Sometimes it is unable to eat steadily due to predators.

The earthworm has both male and female reproductive organs and each will produce both sperm and eggs. They are hermaphrodites. The two earthworms lie head to tail and exchange sperm, thereby cross-fertilizing each other. Eggs and sperms are placed in little egg cases that are left in the soil. After hatching the young worms leave the egg cases.

Earthworms are very important to the soil as the worm digs through the soil by eating dirt. The soil will pass completely through the worm and out of the anus. There will be bits of humus in the soil digested by the earthworm, this process loosens the soil and allows the air and rain to enter, improving the growth of plant roots. The wastes excreted by the earthworm are mixed with the soil thereby making soil rich in minerals.

There are also sandworms that are segmented worms. Actually they are much more active than the earthworm. They will burrow to capture other small animals with pincerlike jaws. There are many little flaps on each side of the body that enable them to swim and they will absorb oxygen from the water as these flaps form a sort of respiratory system.

The sandworm has four eyespots and a group of tentacles on its head making them sense organs. These sense organs are more developed than in the earthworm.

Leeches are segments worms and are often called "blood suckers" because they have the ability to get on the skin of swimmers and suck blood. A leech has a suction cup at each end of the body. Inside the mouth are three small, hard jaws that are used to cut the skin of the host so blood will flow. Most leeches are found in fresh water.

As you can see segmented worms are fairly complex especially the earthworm. They also have well-developed organs and systems. Segmented worms are found in soil and also in fresh water. On every square yard of many lakes and streams you will find hundreds of segmented worms.

It is necessary for them to have their front ends in the mud usually and their back ends waving in the air to get oxygen.

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