christmas-tree-worms.jpgANNELID.jpg ANNELIDS
Daisy Joo

Kingdom: Animalia
: Annelida

Marine Worm Christmas Tree Worms

Annelids are worms recognized for their segmented bodies; in fact, annelida, the name of this phylum, means “little rings”. Though the annelid’s little rings look like an external characteristic, the segmentation occurs internally as well. In fact, the segmentation allows for different body regions to modify for different functions; these body sections specialize. This regional specialization is a significant evolutionary adaptation developed in this phylum. Annelids have three body regions, which are mostly made up of repeated units called segments. Each segment generally contains organs involved in excretion, locomotion, and respiration. The Segments are formed sequentially with the youngest segment at the posterior end of the body. Chaetae or setae, mini hair like structures projecting from the surface of animal tissue that covers the body, are another distinguishing feature of annelids. Chaetae perform various functions involving secretion and transport of materials(6)(BMB). The partitions which separate the inside cavityfluid-filled, the coelom, of the worm into segments are called septum.

There are around 15,000 annelid species which vary in length, habitats, and even how they acquire food. For this reason, scientists have organized annelids into smaller groups called “classes”. The phylum Annelida includes 3 different classes called, Oligochaeta, Polychaeta, and Hirudinea. Although these names may sound foreign, you have probably encountered a number of worms from these classes.

  • Class Oligochaeta
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    [14 Nangia]
    In the class Oligochaeta are earthworms and their relatives, including worms that reside in freshwater. The main characteristics of this class are a reduced head and the absence of parapodia (pairs of leg-like and fleshy appendages extending from the body).

  • Class Polychaeta
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    [15 Nangia]
    Most of the worms in this class live in water; they can live in the seafloor, drift along the surface of water, and even live inside tubes that they create. Polychaeta have well-developed heads and parapodia with attached setae. Setae are stiff hair-like bristles which are attached to the feet-like appendages called parapodia.

  • Class Hirudinea
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    [16 Nangia]
    This class includes leeches, both fresh water and land dwelling. The bodies of the leeches are normally flattened and do not have the bristle-like setae. The coelom, the fluid-filled cavity inside worms' bodies, and segmentation are reduced in Hirudineas. Hirudineas also are known to be hermaphrodites, which means that they have both male and female sex organs. Although they have the ability to reproduce alone, they still mate with another member of its species. (JAC) [12] There is not much that separates Hirudinea, or leeches, from the other classes of annelids; however, they do possess a slightly different musculature from that of other worms. They posses the circular and longitudinal muscles of the other groups, but also have a set of diagonal and dorsoventral muscles, extending from the "back" to the "abdomen." This is what results in the reduced coelom and loss of septa, which separate the coelom at segmental junctures, as this space is taken up by the extra sets of muscles. Setae, which are used by other worms to anchor themselves while burrowing, are absent in the leeches. These structural changes are reflected in leech locomotion. Leeches crawl and swim, but are not capable of burrowing. In order to crawl (often called looping) the leech uses its anterior and posterior suckers to alternately anchor itself to the substrate. A site notes that all leeches are hermaphroditic. However, the female and male reproductive organs are not active at the same time in the lifecycle, meaning they must find a partner with which to copulate, or mate. They can, however, simultaneously exchange sperm with a partner and store it for later use. Unlike other annelid groups, leeches cannot reproduce asexually or regenerate damaged body segments. (18) (SS)

Acquiring and Digesting Food
Annelid’s digestive system includes the pharynx, the esophagus, the crop, the gizzard, and the intestine. Though the septa divide the body cavity into sections, the digestive tract runs the entire length of the worm. The use of this digestive tract allows annelids to be able to continually digest food and still crawl,wiggle, and squirm as they digest.(7, Grace Rehnquist)

The earthworm of the Class Oligochaeta specifically eats soil; they subsist by obtaining nutrients from the dirt as it passes through the digestive tube. In fact, farmers appreciate worms because they cultivate the soil(11 VM). They eat dead and decomposing leaves, decaying roots, and other bits of plant material, or detritus found in the soil(11 VM). Worms living in leaf litter layer, upper soil, or in soils in tree canopies eat freshly dead plant materials (11 VM).The fanworms, marine annelids of the Class Polychaeta, acquire their food with their hairy tentacles. By extending these feather-like appendages, the fanworms can trap tiny food particles. Leeches, annelids of the class Hirudinea, are parasites, predators and scavengers. Most leeches feed on invertebrates, but there are some which suck blood of other animals (including humans!). Some parasitic leeches acquire their food using sharp jaws to cut through the skin of the host, and some use digestive enzymes which cut through the skin. Other food acquisition tools these parasitic annelids have include an anesthetic used to numb the host and a chemical called hirudin used to stop the blood of the host from congealing. (1)

Sensing the Environment
Earthworms have a pseudo-brain, a fake brain; it is a pair of cerebral ganglia which is on top of the pharynx of the annelid. Ganglia are nerve tissues which transmit information to other nerves and sensory organs (4 SES). These sensory organs include the palps (extended appendage that can be used for feeding in some species), antennae (usually located at the head of annelids), eyes, statocysts (organs that help annelids balance), nuchal organs (ciliated and paired organs, innervated from the posterior part of the brain), and lateral organs (responsive to light and touch, i.e. setae) (4 SES). Nerves are wrapped around the pharynx and are connected to a subpharyngeal ganglion. From the subpharyngeal ganglion, nerve cords are attached and run the length of the worm. These nerve cords penetrate the segmenting septa to run the length of the worm, but to solve the issue of an encompassing nervous system, the nerve cords are fused with segmental ganglia. (1)

Because annelids do not have legs or arms with which to move around, they have other ways of moving (10 BL). Many annelids have setae, bristles that allow worms to burrow by creating traction. By using the setae to anchor themselves in the ground, annelids then use their body muscles to move themselves (10 BL). In polychaetes, which means “many setae”, the setae are attached to the parapodia. Every segment of polychaetes has a pair of these parapodia which allow for movement as they act similarly to feet and paddles. The attached setae are made of polysaccharide chitin. Earthworms have four pairs of setae on each segment for movement, but also have more modes of locomotion; earthworms, as well as many other annelids, move by using two muscles. These muscles, longitudinal and circular, are coordinated so that they work against the coelom’s fluid, a noncompressible fluid; this way annelids may inch along. The coelomic fluid creates what is named “a hydrostatic skeleton” . The skeleton does not support the body with a set of bones, instead the annelids are supported by hydrostatics, the pressure of the coelomic fluid. (1) Metamerism, the repetition of a similar body part, helps annelids move more easily because the muscle contractions are localized [HZ]. This means that the muscles of each segment only have to make that segment move, instead of the entire length of the worm, for example (17) [HZ].

Instead of lungs, annelids use their skin as their respiratory organ. The skin of the annelids has a large number of tiny blood vessels which allow for the annelids to “inhale” oxygen and transfer the oxygen through the bloodstream. In polychaetes, the marine worms with many setae, the parapodia are abundantly distributed blood vessels and act like fishes’ gills. So, not only do the parapodia help annelids move, these lateral flaps of flesh of the polychaetes are also used to exchange respiratory gases with the surrounding water. Because annelids use their skin as their respiratory organ there needs to be contact between the skin and moisture to intake oxygen; this explains why most annelids reside in water and damp soil. (1)

Metabolic Waste Removal
Each segment of the annelids has a pair of tubes called metanephridia. These tubes are used in excreting metabolic wastes through pores in the skin. The wastes are extracted from the blood and the coelomic fluid by funnels called nephrostomes. These funnels are ciliated, meaning they have tiny hair-like projections which help in extracting the metabolic wastes. In earthworms, undigested materials are excreted through the anus as castings. (Darwin once estimated that 1 acre of British farmland had worms that excreted 18 tons of castings per year.) (1)

The circulatory system in an annelid is usually closed, which means it is within well-developed blood vessels; however, in some polychaetes and leeches the circulatory system is partly open, which means the blood and coelomic fluid mix directly in the fluid of the body cavity. The circulatory system of annelids includes a network of vessels which passes blood around all the inner organs and blood containing hemoglobin which carries the oxygen to be distributed to all parts of the annelid. It is also said that some worms carry a green oxygen-carrying pigment, and others have unpigmented blood. Blood is distributed to each part of the body by lateral vessels; it flows toward the head through a contractile vessel above the gut and returns through vessels below the gut. Some of the lateral vessels contract to pump organs for driving the blood (3. Sharma). In an earthworm, the two main vessels are the dorsal and ventral vessels. These vessels run along the length of the worm and are connected by segmental pairs of vessels, pairs of vessels tracing the circular width of the worm (see image). The blood is pumped throughout the circulatory system by the muscular vessels: the dorsal vessel and the 5 pairs of vessels circling the distance around the esophagus. (1)

Anatomy of an Earthworm

Anatomy of an Earthworm

Self Protection
The annelids protect their internal organs through the aforementioned hydrostatic skeleton. The coelom, the fluid-filled body cavity, protects the annelid’s innards by the padding and cushioning of the coelomic fluid pressure. (1) It is also used to store and protect the gametes, female eggs or male sperm, and is designed as a series of compartments. This allows some of the coelom to stay intact and to not lose all of the content of the coelom even if the worm is injured.(HS,5) In the hydrostatic skeleton, pressures created by muscle contractions maintain the shape of the annelid. Earth worms use the hydroskeleton to change their shape as they push themselves forward. (AR, 13). In terms of self-protection against predators, earthworms living deep in the soil are rather limited for defensive behaviors(11 VM). Those living closer to the surface, however, either retreat into their burrows, or they jump and thrash about sometimes even purposely breaking off a few tail segments to escape (11 VM).

Osmotic Balance
Because annelids can use skin in gas exchange, the skin is moist and susceptible to losing large amounts of water. For this reason, annelids tend to reside in moist and damp earths and freshwater. (1)

Temperature Balance
Annelids are exothermic, which means they get their body temperature as a result of their surroundings. Basically they have no system to regulate their temperature; they are the temperature of their habitat. (2) Sarah Vlach Because they are the tempeature of their habitat their behavior can regulate their tempeature balance meaning when a worm is cold it will move closer to the surface and when its too hot it will travel deeper in the ground. (SJB)(1)

annelid reproduction

While humans can be either male or female, annelids are both. Because they have both male and female gametes (reproductive cells like eggs and sperm), annelids are labeled as hermaphrodites. Earthworms are hermaphrodites, but they can reproduce both sexually or asexually with each other. Reproducing sexually involves the reproductive cells, the gametes; reproducing asexually does not involve the use of gametes.
Earthworms can reproduce sexually by aligning their bodies in a certain position. This position allows for the earthworms to exchange sperm with one another. Once they have exchanged their male gametes, they separate, and the sperm cells inside the earthworms become stored temporarily. Meanwhile, a gooey cocoon is released by a certain organ called the clitellum. This cocoon brings together the exchanged male gametes with the already possessed female gamete eggs. The cocoon slides down the worm to pick up the eggs and the stored sperm and eventually reaches the head of the worm. Here, at the head, the cocoon comes off and buries into the soil. In the soil, the embryos for the worms develop.
Uniquely, some of these hermaphrodites can also reproduce asexually. There are certain earthworms that can reproduce by splitting their selves and reforming! This is called reproduction by fragmentation followed by regeneration. (1)

Importance of Annelids to ecology (RK)

Although not largely appreciated by society, annelids play a great role in our environment and ecosystems. They are essential for agriculture in that they recycle and aerate (loosen and add oxygen) to soil. In additions to allowing air into the soil, their burrowing activities also provide food for growing plants by mixing soil with bits of plant materials(11 VM). Several species of earthworms are raised to collect their waste for use as a high-quality compost for gardening (11 VM). They also help scientists to measure water quality (freshwater and marine) as they are indicators of oxygen content, salinity, organic pollutants and metal concentration. Annelids also play a role in the global food web, since fisherman use them for bait (8). They also play a large role in the marine food chain and support the abundance of fish that humans eat (9).
Vulnerable Species: Driloleirus americanus, Driloleirus macelfreshi, Komarekiona eatoni, Megascolides australis
Endangered Species: Mesonerilla prospera, Phallodrilus macmasterae (9)

Review Questions

1) How do Annelids conduct locomotion? (NK)
2) What do Annelids use instead of lungs for respiration? (RJS)
3) What benefits do annelids bring to our environment? (ER)

Edited by:
Sarah Vlach
Yasheka Sharma
Sarah Schwarzschild
Hilary Stepansky
Brittany Marcus-Blank
Grace Rehnquist
Rachel Kornetsky
Becca Levenson
Vonai Moyo
Josh Czik
Arielle Reiter
Sam Blatchford
Jesse Carmen
Meru Nangia
Hanna Zhu
SSEthan Richman
Works Cited
(1) Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston: Benjamin-Cummings Company, 2002.
(3)23 Nov. 2008 <>
(4) Rouse, Greg W., and Fredrick Pleijel. "Characteristics of Annelida: Plesiomorphies and Other Features." Tree of Life. 2002. 2 Dec. 2008 <>.
(6 ) Rouse, Greg W., and Fredrick Pleijel. "Annelids." Tree of Life. 2002. 2 Dec. 2008 < >
(7) "The Shape of Life." Phylum Annelida. 2002. PBS. 3 Dec. 2008 <>.
(8) Cracraft, Joel, Michael J. Donoghue. Assembling the Tree of Life. New York: Oxford University Press, 2004. (via Google Book Search preview)
(9) Ramel, Gordon. "The Phylum Annelida." Earth-Life Web Productions. 29 Sept. 2008. 7 Dec. 2008 <>.
(10) "annelid." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 07 Dec. 2008 <>.
(11) 7 Dec 2008 <>
(12) "Leeches." 2003. Australian Museum. 7 Dec. 2008 <>.
(13) "Skeletons." Biology Daily. Dec 7 2008. <>.
(17) Myers, P. 2001. "Annelida." Animal Diversity Web. 13 December 2008<>.
(18) Hebert, Paul D. "Annelida (Aquatic)." The Encyclopedia of Earth. 20 Aug. 2007. Boston University. 17 Dec. 2008 <>.
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eview Questions:
1. How does the hydrostatic skeleton aid in protecting an annelid? (Jesse Carmen)
2. How are annelids able to move through their environments? (DP)