Earthworm (annelid)
Originally by Justin Wilbur
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1. Classification/ Diagnostic characteristics
All annelids, including the Earthworm, may be easily identified by the visible segmentation. Segmentation allows annelids to move their body parts independently, which is why they tend to be long, tubular and very flexible. Many annelids have two or more eyes, although Earthworms, specifically, do not. Earthworms and other annelids also lack a rigid internal or external skeleton, and instead they have a thin, permeable body wall that gives them their smooth and slimy texture. Earthworms must live in warm, damp environments, which is another identifying trait. In some annelids, but not in the Earthworm, tentacles, lateral body outgrowths called parapodia, and stiff bristles called setae that grow off of parapodia. Earthworms do have small setae between their ridges, although they are difficult to observe.[i]

Some earthworms are semi-aquatic or aquatic

Biology and Ecology of Earthworms By Clive A. Edwards, P.J. Bohlen


external image EarthwormAnatomy1.jpg
http://www.sas.upenn.edu/~rlenet/Earthworms.html

2. Relationship to Humans
One group of annelids, used to be used in medicine as a way of removing “bad blood” from an infected individual. One type of leech, Hirudo medicinalis, is still used today to reduce fluid pressure and prevent [[#|blood clotting]] in damaged tissues, to eliminate pools and blood and scarring. Some other leach species contain anesthetics and blood vessel dilators.[i]
Earthworms themselves are often not used outside of food for other test subjects, but they have long been man’s choice as fishing bait!
Earthworms are a key component to the process of composting and organic material production.

http://www.vermidirtfarms.net/Earthworm_Info.php


Because earthworms are decomposers, they are very important in growing gardens. They break down the dead into the soil, allowing new organisms to grow under the fertile garden. Earthworms let air into the ground, mix up minerals, and help drain the soil of water. Additionally, some people keep them as pets, though this is not very common.
http://www.fcps.edu/islandcreekes/ecology/earthworm.htm (BHu)


Humans use earthworms for the remediation of organic "waste" materials, reducing the pressure of landfills, and aiding in the regeneration of our top soils. Humans depend on these worms to help incorporate mineral matter into the topsoil layer as well as aerating and mixing the soil through their movement and feeding patterns so the soil.
http://www.vermidirtfarms.net/Earthworm_Info.php

Earthworms are a source of food for numerous animals, like birds, rats, and toads, and are frequently used in residential composting and as bait in commercial and recreational fishing. Their numbers are strong throughout their range—they’re even considered [[#|agricultural pests]] in some areas—and they have no special status.
http://animals.nationalgeographic.com/animals/invertebrates/earthworm/

Speaking of food for organisms, worms are actually a very common delicacy for humans. Over 90 countries eat insects and worms. In Asia, Africa, and Latin America, people regularly eat worms. Even some restaurants in the United States serve worms to eat. Worms give considerable protein, calcium, iron, magnesium, potassium, phosphorous, and copper; they provide nutrition comparable to eggs and cow's milk. Worms have also been used medicinally, as they have been given to those suffering from malaria and women who recently gave birth.

Worms can also be pests. Roundworms, for example, are parasites that live and feed in a person's gut. People get roundworms because of poor sanitation and hygiene, usually in warm, tropical areas.

http://www.livestrong.com/article/472699-worms-as-a-food-source-for-humans/ (BHu)


3. Habitat and Niche
Earthworms occupy the soil environments of most temperate areas that have sufficient moisture, but are not flooded. Many other annelids; however, are adapted to marine and freshwater life as well. Annelids often occupy the niche of an infrequently hunted decomposer in that they tend to thrive but avoiding detection and exchanging nutrients directly with the abiotic environment, specifically by eating and digesting soil.[i]

"They are indigenous to Europe, but are now abundant in North America and western Asia." This is a really helpful link for a lot of fun facts and specifics on the earthworm. I would also add in "Niche" their place in the food web. "Earthworms are a source of food for numerous animals, like birds, rats, and toads, and are frequently used in residential composting and as bait in commercial and recreational fishing."

http://animals.nationalgeographic.com/animals/invertebrates/earthworm/



4. Predator avoidance
Earthworms tend to live in damp soil, which naturally protects them from landborne predators. Because Earthworms are rarely hunted underground, they are generally safe unless forced out of the soil by flooding or other disturbances. In these cases, Earthworms are generally defenseless to predators.[i]

external image p15485pc.jpg
Earthworms exposed to predators due to flooded conditions. They are unsafe and not doing a good job at avoidance. (Colin Gray)
http://www.teara.govt.nz/en/earthworms/1/1



Here, we see a ruthless woodcock prey on a poor earthworm (Prashant)
external image Woodcock_earthworm.jpg

Although they don't really have any physical attributes that can act as defenses, behavior-wise, if/when an earthworm is attacked by a predator, they thrash wildly to try and escape and also sometimes are able to produce a repulsive odor. Generally these behaviors don't have that much of an affect, but the earthworm is also capable of regeneration (dependent on how much of earthworm was removed and where parts were removed from). (http://www.ehow.com/how-does_5166642_do-earthworms-protect-themselves.html)

5. Nutrient acquisition
external image earthworm-digestive-system1.jpg
The Digestive system of an Earthworm (GC)
http://scienceprojectideasforkids.com/2010/earthworm-digestive-system/


Earthworms and other annelids acquire all of their necessary airborne nutrients, such as oxygen, by diffusion through the thin walls of their bodies. Earthworm worms gain the remainder of their nutrients by constantly eating the soil in front of them through their mouths, passing it through their guts, extracting nutrients, and passing out remaining soil through the anus. Other annelids may also have tentacles that serve to help catch prey or [[#|filter]] food from the surrounding water.[i]
Earthworms have strong mouth muscles which helps them eat their food because they do not have teeth. Nightcrawlers come to the surface of the soil at night to pull leaves to eat ito their burrow. Aside from soil, they eat living organisms such as nematodes, protozoans, rotifers, bacteria, fungi as well as the decomposing remains of other organisms.
http://www.biologyjunction.com/earthworm%20facts.htm#eat

6. Reproduction and life cycle
Not all annelids, but all oligochaetes (including Earthworms), are hermaphroditic, meaning that they are both male and female. Sperm is still exchanged between two individuals of the species by being excreted onto the other’s body. This process allows for genetic combination but eliminates the need of finding an opposite ses mate. The clitellum then secretes a cocoon which is shed. The cocoon then begins fertilization and when complete, several new worms are born and immediately begin independent life.[i]

worm_cycle%2520rick.jpeg
http://wiggos.com/Schools.aspx?pid=90

7. Growth and Development
Earthworms and other annelids do much of their growth within their cocoon following fertilization, and are independent and almost fully grown when their cocoon sheds.[i]
Cocoons can take up to 11 weeks to fully hatch and can contain around three worms per caccoon.Typically it takes an earthworm about two to three months to mature completly. (http://www.eulesstx.gov/composting/vc_reproduction.htm)




8. Integument
The Earthworm is covered by a thin, permeable body wall that allows for easy gas exchange but little protection. One annelid group, pogonophorans, secrete tubes that are made out of chitin in which they live for protection.[i]



9. Movement
All annelids are characterized by the segmentation in their bodies that allows for many parts to work independently. This mechanism allows annelids, including the Earthworm, to slither through soil and water alike through the easiest possible rout. The Earthworm is one of many organisms with a hydrostatic skeleton, which means that a volume of fluid is surrounded by a muscle that contracts to create a moving bulge within the worm. The bulge helps to force the worms along and give it motion. Some annelids also have tentacles, and setae that help to move and provide traction for the organisms. Earthworms have setae but no tentacles.[i]

An earthworm moves by peristalsis, so essentially it moves by contractions of muscles. The elongation and shortening of the segments allows the earthoworm to wiggle at a slow pace. (Cam Somers) http://classic.sidwell.edu/us/science/vlb5/Labs/Dissection_Lab/conclusions.html




10. Sensing the environment
Earthworms can't see forms or shapes, but they do have photoreceptors scattered across their body which can sense changes in light intensity. Earthworms also have mechanoreceptors (sensitive to touch) and chemoreceptors (sensitive to chemicals) on their surface.
http://www.backyardnature.net/earthwrm.htm

The earthworm’s cerebral ganglion helps to act as a brain that helps form the basis of the earthworms nervous system. The earthworm has a ventral nerve chord attaching to the ganglion that helps to aid the sensing of mechanoreceptors and chemoreceptor’s.-http://www.wigglyworld.org/earthworm-anatomy/


11. Gas exchange
Earthworms and most other annelids have thin permeable body walls that allow for natural diffusion of gases into the body without a process of active gas exchange. This allows for the Earthworm to minimize the energy required to acquire and give up important gases for life. Some annelids (excluding the Earthworm) have lateral segments extending from their body walls called parapodia that serve to increase surface area and better facilitate gas exchange.[i]

As stated above earthworm's gas exchange happens through their body walls. The Oxygen they absorb dissolves due to the moisture on their skin and then travels into the skins blood, capillaries. Hemoglobin carries oxygen in their blood. As stated above CO2 diffuses out of their skin.
http://www.porcellio.scaber.org/Eworms/eworm.htm



12. Waste removal
Earthworms and most annelids remove waste by passing it directly out through their body-spanning intestines and into their wake, be it water or soil. Earthworms have internal muscles that expand and contract to reach expel waste. Each segment of the Earthworm has a pair of metanephridia which begins with a funnel like opening called a nephrostome, continues as a tube, and ends in a pore called a nephridiopore. Coelomic fluid is funnel through the tubules, during which the cells absorb some molecules and secrete others, producing a diluted nitrogenous waste that is carried out of the body.[i]

There are two nephridia, what excrets nitrogen, in each segment of the body.

http://www.porcellio.scaber.org/Eworms/eworm.htm



13. Environment physiology
The Earthworm tends to live in temperate areas with moderate rainfall and temperature. This is because they regulate water and oxygen levels through diffusion through their body walls, and are thus very sensitive to external conditions. The Earthworm exchanges other nutrients, such as salt, by consuming it from the soil, passing it through its body, and excreting it in its wake as it moves.[i]

external image earthwormcirculatory.gif

14. Internal circulation
Annelids contain coelom, large sacks within the organism through which nutrients flow. Earthworms utilize muscles within their bodies that expand and contract to move nutrients through their bodies. To control blood flow, annelids have multiple hearts that pump blood through a system of closed blood vessels. This means that blood flows separately from the other fluids around it. The Earthworm has a ventral blood vessel that carries blood from the worm’s anterior to its posterior end, and which branch off into small blood vessels along the way to ensure circulation throughout the system. Some blood is diffused due to pressure into the coelom, where it is removed through the anus.[i]

Circulation in the earthworm is through a series of closed vessels. In other words, it has a closed circulatory system. The two main vessels are the dorsal and ventral blood vessels. These vessels are the main pumping structures. In the dorsal vessel, blood moves anteriorly (to the front of the body). The dorsal vessel is a dark line running along the dorsal surface of the digestive tract. In the ventral vessel, blood moves posteriorly (towards the back of the body). Segmental branches off the ventral vessel supply the intestine and body wall with blood. These branches eventually break into capillary beds to pick up or release nutrients and, oxygen. Gas exchange occurs between the capillary beds of the body surface and the environment. Oxygen is carried by the respiratory pigment hemoglobin, which is dissolved in the fluid portion of the blood. From these capillary beds, blood is collected into larger vessels that eventually unite with the dorsal vessel. At the level of the esophagus, segmental branches are expanded into five pairs of aortic arches, or what have been called "hearts". They are dark, expanded structures on either side of the esophagus. Although these are contractile, they only function in pumping blood from the dorsal to the ventral vessels.

Taken from the website:
http://www.esu.edu/~milewski/intro_biol_two/lab__12_annel_arthro/Annelida.html
A diagram of the Earthworm's circulatory system. Picture taken from:
http://www.sas.upenn.edu/~rlenet/Earthworms.html


15. Chemical control (i.e. endocrine system)
The earthworm worms endocrine consists of a hydrostatic skeleton which entails that an earthworm is mainly comprised of made of fluid. Also, in the endocrine system there is a fluid-filled sac where all the body organs are contained and working. (Evan Kates) (http://classic.sidwell.edu/us/science/vlb5/Labs/Dissection_Lab/conclusions.html)

16. Review Questions

1. Describe the life and reproductive cycles of an earthworm
2. Earthworms are one of many species of animals that are hermaphroditic, meaning that they are able to do this?
3. Explain the process of internal blood circulation in earthworms using the terms closed circulatory system, dorsal and ventral blood vessels, capillary beds, and aortic arches. (Alexander Soloviev)
4. Through what bodily mechanisms does the earthworm acquire necessary nutrients to survive? Make sure to include an explanation of gas exchange.
5. Is there any case in which earthworms are defenseless against predators? If so explain.
6. Explain why an earthworm's circulatory is frequently studied and compared to

References:

[i] Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sudnerland, MA: Sinauer Associates, 2012. Print.