Annelida

**Annelida** Segmented Worms By Marissa Lown-Klein There are 3 classes of Annelida and about 15,000 species that range in length between 1mm to 3m. The largest recorded annelida was a //Michrochaetus michrochaetus// which measured 6.7m long. [3] (LJ) The three classes are **Polychaeta**, **Aelosomata**, and **Clitellata.** The Polychaete class contains the largest number of species of Annelidas. [|3] (MP) Certain polychaetes are sequential hermaphrodites (they start off their life as members of one sex and later on convert to the other sex). [|3] (NI) The Aelosomata encompass around 25 different species of Annelida. They live in both terrestrial and marine biomes. They are hermaphrodites, possessing both male and female reproductive systems. (RG) [15]. Many Anelida live in the sea and usually on the sea (fresh water) floor, some however do inhabit damp soil. Annelida have various ecological roles: some are parasitic and others filter-feed or prey on other invertebrates. The most significant role of the Annelida is reworking of soil and sediments. (AC)(8) media type="youtube" key="DZig6EL5B6A?fs=1" height="385" width="480" (LJ) **__Characteristics__** Annelida have segmented bodies that resemble a series of fused rings. The Segmentation is of the worm is called the **Septa**. The Septa allows for high degree of specialization of each separate body region. Even though the Septa separates the worn into different parts, the digestive tract, blood vessels, and nerve cords all go through the septa and run along the body posteriorly. They are Coelomate creatures, more specifically **Protostomia** (Coelom from cell masses)**.** **Coelom** provides a **Hydrostatic Skeleton** for the worm. A Hydrostatic Skeleton is fluid that is held under a lot of pressure in a close body compartment, which allows the organism to use muscles to control the shape of its body. The Worm contracts and relaxes to change the shape of the fluid compartment, allowing for movement. Annelids can change the shape of each segmented part of their bodies which allows them to move using peristalsis (movement due to muscle contractions). A positive thing about this is that this helps cushion the internal organs from shock. The Hydrostatic Skeleton allows for a new and divers method of locomotion, provides body space for storage and for complex organ development. This also cushions the organs to protect them from internal struggles, allows functional separation from the action of the worms movements and the organs movements inside the body. Below is an Image of an Annelida sample. Annelids are bilaterally symmetrical, which means the right half of their bodies are mirror images of the left half. Annelids vary significantly in size, ranging from less than 1 mm in length to more than 3 m. The smallest known annelids are the Neotentrocha, which are a few hundred microns long. Among the Hirudinea class, the largest leech in the world is the Haementeria ghilianii, which is more than 40 cm in length and lives in South America. (MT) (10) The body of Annelids is covered by an external cuticle that is never shed or molted. This outer covering does not contain cells, but the skin under the cuticle secretes fibers that are collagenous and contain scleroprotein. The cuticle is formed from these secretions. **Chaetae** (stiff hairs) are apart of the cuticle. They contain large amounts of chitin. Under the cuticle and skin, lies a circular muscle. This muscle forms a continuous sheath around the body. (LW) (19) Some annelids, such as leeches, molt meaning they shed parts of their body off, usually an outer layer of skin, while other annelids typically do not. Polychaetes, a class of annelids are an exception to the characteristic of "non-molting," some polychaeta molt their jaws. (cw)(22) The annelids have a central nervous system. Their nervous system consists of a dorsal brain and a pair of nerves on either side of their esophagus. They also have paired ventral nerve cords that have paired ganglia in each segment. Finally, the annelids have transverse nerves that attach the ganglia in each of the annelid’s segments. (21)(MF)

Annelid locomotion would not be possible without setae (sometimes called chaetae). Setae are hair-like projections that are made of chitin and come off of the annelids entire body. They allow annelids to grip surfaces, which is essential for burrowing, and increase annelid surface area, which helps aquatic annelids swim. Setae themselves are able to move in and out of an annelid's body wall. (CSR, [|16], [|17])
 * __Locomotion__**
 * Annelids have 2 muscle sets
 * **Elongating muscles**- circular muscles
 * causes the worms body to be elongated and thin
 * **Shortening muscles**- longitudinal muscles
 * causes the worms body to be short and thick
 * [[image:http://occawlonline.pearsoned.com/bookbind/pubbooks/campbell6e_awl/medialib/assets/e-book/fig/big/fig49-27.jpg height="360" caption="external image fig49-27.jpg"]]

To move, annelids first contract their longitudinal muscles which relaxes the circular muscles. At the same time, the setae come out. Next, the circular muscles contract and the longitudinal muscles relax. The setae are then withdrawn and the annelid is pushed forward. (20)(SM)


 * __Digestion__**
 * The worms digestive system consists of the
 * pharynx
 * esophagus
 * the crop
 * the gizzard
 * the intestines
 * The earthworm eats dirt, digesting the plant and animal matter, and eliminates the rest. Food goes through the esophagus and is stored in the crop. The gizzard grinds the food and then the food passes through the intestines, which keep nutrients and eliminate waste through the anus. (DB) (4)
 * Some of the worms have developed full jaws to snatch prey and grasp matter, some have a reversible pharynx to envelope food, and others use cilia to wash food particles into their mouths. (JS) [|(8)]

(18 NG)

Segmented Worms have oxygen-transporting blood, also known as blood that contains Hemoglobin. They have dorsal and ventral pairs of vessels connected by segmental pairs of vessels.The first 5 pairs of segmental vessels are known as the **Axillary hearts**, which act as a heart and propel the blood ventrally.The Dorsal vessel and the first 5 pairs are very muscular and act as pumps to pump the blood through the rest of the body around the esophagus. The segmental branches of the ventral vessel carry blood to the annelida's intestines and body wall. These branches then break up into capillary beds that exchanges nutrients and oxygen with the outside environment (KS)4 The Right half of the image shows the closed circulatory system in the Annelids.
 * __Circulation__**

There are tiny blood cells all over the skin of Annelids that diffuse oxygen between the worm and the environment. "The annelid's body contains a double transport system of which coelomic fluid and the circulatory system carry food, wastes, and respiratory gases. Flatworms and annelids use their outer surfaces as gas exchange surfaces. Arthropods, annelids, and fish use gills; terrestrial vertebrates utilize internal lungs. Gills are very efficient at removing oxygen from water: there is only 1/20 the amount of oxygen present in water as in the same volume of air. Water flows over gills in one direction while blood flows in the opposite direction through gill capillaries. This countercurrent flow maximizes oxygen transfer." [M.S.] [|2]
 * __Respiration__**

In each segment part of an Annelida there is a excretory organ call the **Metanephridia.** They begin in the segment before with the **Nephrostome,** which is a ciliated funnel like object that receives Coelomic fluid wastes and blood. Then the fluid goes through the **collecting tubules** that is covered by a capillary network. As the waste moves along the collecting tubule the capillaries reabsorb most solutes and return them to the blood. Nitrogen waste stays in the tubule and is dumped out the **Nephridiopore,** an external pore that is used just to discharge Urine. Below is a picture of each part of the system.
 * __Excretion and Osmoregulatory Functions__**

The brain of an Annelida is made up of a pair of **Cerebral Ganglia**, which sits above and in front of the pharynx. The nerves loop around the pharynx and connects to a sub-pharyngeal ganglion, then a fused pair of nerve cords runs posteriorly along the worms body. "Their brains vary in structure. For example, mobile active form of Annelids have more complex brains, while sessile or burrowing forms have simple brains with little differentiation. The brain is connected to the ventral nerve cord which is made up of pairs of cords that are bound together and run the length of the body. The ventral nerve cord varies in thickness. It dilates into a ganglion in each segment, from which pairs of segmental nerves pass to the body wall, muscles, and gut." (AK) (5) Annelids also have a variety of sensory organs. There are six major kinds of sensory structures found in annelids. These sensory organs are the palps, antennae, eyes, statocysts, nuchal organs and lateral organs. Palps and antennae are located on the head of many polychaetes (bristleworms that generally are marine). In some annelid groups they are sensory organs, while in others the palps are used for feeding. Nuchal organs have cilia, are paired, and have chemosensory structures, whose nerves come from the posterior part of the brain. Annelids also have a variety of epidermal sensory cells that may be responsive to light or touch (such as lateral organs). (CC) ([|6])
 * __Brain__**

Segmented worms are Hermaphrodites that cross fertilize. They do mate with other worms, to do so they: Some Annelida fragment then regenerate themselves, a form of Asexual reproduction. All Annelida can also regenerate lost body segments and have natural break points throughout their bodies. Often, many individuals swarm together to mate. Others can bud to reproduce. (KL)([|14])
 * __Reproduction__**
 * align themselves and exchange sperm
 * separate then store the sperm while the **clitellum** secretes a mucosa cocoon
 * The cocoon then slides along the outside of the worms body picking up eggs
 * eventually the cocoon slides off the worms rear and rests in soil with an embryo developing


 * __Food__**
 * Some aquatic annelids swim in pursuit of their food while others are bottom dwellers and burrow in the sand and wait
 * Some annelids are carnivores, eating other small invertebrates (LJ)
 * Others, such as Leeches are parasites, consuming the blood of mammals. (LJ)
 * Other annelids are detrivores. [[|7]] (LJ)
 * Some earthworms known as night-crawlers go grab fallen leaves at night. Since they do not have teeth, they must let the leaf decompose before swallowing it. (GR)
 * When the earthworms build their burrows, they obtain nutrients from the soil that they swallow. (GR)[|23]
 * __Classes__**
 * **__Oligochaeta__**
 * all earthworms and some aquatic species
 * **Characteristics-** **Reduced head; no parapodia, but setae present **
 * Earthworm
 * eats its way through soil.
 * Its digestive tract absorbs all the nutrients it can from the soil as the soil is pushed along
 * undigested materials are mixed with mucosa towards the end of the digestive tract and are excreted through the anus
 * Fun Fact: Farmers love to have earthworms on their fields because they till the earth and their excreted material helps improve the soil texture
 * **__Polychaeta__**
 * **Characteristics-** Well-developed head; each segment usually has parapodia with setae; tube-dwelling and free-living
 * each segment has **parapodia**
 * paddle like structures that aid in locomotion
 * made of polysacharide chitin
 * unjointed, they are divided into upper & lower lobes that end in bristles(ZXU)([|12])
 * the bristles are used for walking,digging, and swimming.
 * in some species the bristles contain poison(for example fire-worms)
 * each parapodia have many blood vessels that act like gills and help with respiration
 * Marine
 * these swim amongst the plankton or crawl on the ocean floor, or even borrow in the sea floor
 * many live in self made tubes
 * made from their mucosa, sand bits, and broken shells
 * Example: Bright Fan Worm
 * extend their leathery tentacles from their tube openings to catch microscopic food particles
 * They are divided into two major types, errantia (free-living), and sedentaria (sedentary). (ZXU)([|13])

(YA)(11)

>>>> Review Questions 1) What composes the hydrostatic skeleton? How does this allow the annelid to move? Do muscles play any role in this type of movement? (SD) 2) Explain the complex and various methods by which annelids reproduce. For each type of reproduction, name another organism that reproduces the same way. 3) Why is or isn't the septa relevant for annelida morphology? Give 2 examples supporting your argument.(TM) 4) Define a Polychaeta and give a specific example. (MC) 5) Describe the pathway of food from intake to excretion. What parts are used, how do they help?(ORS) 6) How do the three classes of Annelids( Polychaeta. Aelosomata, and Clitellata) differ?(ZS) 7. Describe the two muscle sets of Annelids and the role they play in their movement/locomotion. (SI) 8) List the sensory organs an annelid might have and explain what the annelid uses these sensory organs for. (RL)
 * **__Hirudinea__**
 * Leeches
 * ** Characteristics- **Body usually flattened, with reduced coelom and segmentation; setae absent; suckers at anterior and posterior ends; parasites, predators, and scavengers
 * many are freshwater leeches
 * some are land leeches that live in very moist vegetation
 * between 1-30 cm long
 * Most feed on other invertebrates
 * Some feed by attaching themselves to other animals and living off their blood
 * known as parasitic
 * [[image:http://elothtes.pbworks.com/f/1144282538/Annelida.gif height="365" caption="external image Annelida.gif" link="http://elothtes.pbworks.com/f/1144282538/Annelida.gif"]]
 * Parasitic leeches have a special way of getting the blood
 * Some use blade-like jaws to slit the skin while others secrete enzymes that create holes in the skin
 * The host usually does not feel a thing because the leech also secretes an anesthetic
 * Next the leech secretes a chemical, **Hirudin**, that keeps blood from coagulating/ clotting
 * then the leech sucks as much blood as it can hold, usually 10 times its weight, and will live off this blood for a couple of months
 * Fun Fact- Once leeches were used by physicians for blood letting, they are now used to treat bruied tissues and to stimulate circulation on reattached fingers and toes

Source:

1. Campbell, Neil A, and Jane B. Reece. Sixth Edition. San Fransisco: Pearson Education, Inc, 2002. 2. [|http://www.cartage.org] 3. [] 4. [] 5. [] 6.http://tolweb.org/articles/?article_id=57 7. [] 8. [] 9. [] 10. [] 11. www.ib.unicamp.br/Library/polychaeta_da1.jpg&imgrefurl=http://www.cnbaforo.com.ar/showthread.php 12. http://www.ucmp.berkeley.edu/annelida/polymm.html (ZXU) 13.http://www.ucmp.berkeley.edu/annelida/polysy.html (ZXU) 14. [] 15. [] 16.http://userwww.sfsu.edu/~biol240/labs/lab_17hydrostaticsk/pages/annelid.html (CSR) 17. http://www.buzzle.com/articles/annelida-characteristics.html (CSR) 18. http://yhsbiology.wikispaces.com/Annelida+Videos (NG) 19. [] (LW) 20. @http://userwww.sfsu.edu/~biol240/labs/lab_17hydrostaticsk/pages/annelid.html (SM) 21. http://facstaff.gpc.edu/~apennima/Principles/Annelida.ppt (MF) 22. <span style="font-family: 'Calibri','sans-serif'; font-size: 11pt; line-height: 115%;">http://en.wikipedia.org/wiki/Annelid#Distinguishing_features <span style="font-family: 'Calibri','sans-serif'; font-size: 11pt; line-height: 115%;">23. http://www.biologyjunction.com/earthworm%20facts.htm#food (GR)