Rachael Judson
external image caulleriella_jpg.jpg

Diagram of Annelid anatomy (CM)
Diagram of Annelid anatomy (CM)

Earthworms, Leaches, and Fanworms OH MY!
Annelida, meaning little rings, is the group made up of segmented worms. Phylum Annelids contains around 15,000 species. They range in length from 1milimeter to 3 meters in length. It is made of three major classes: oligochaeta, polychaeta, hirudinea.
The subclass Oligochaeta are the second most numerous group of annelids with around 3,100 species. Oligochaeta live in marine, freshwater and terrestrial habitats. Generally they have a more rounded cross-section, a less distinct head and are less diverse in form than the Polychaetes. They are normally hermaphrodites, and possess a clitellum as adults, an organ which looks like a bandage of skin wrapped around the animal. This clitellum, from which the whole group takes its name has an important function in sexual reproduction, otherwise reproduction may asexual by fission. A few species are parasitic but most species are free living.(RW)
The subclass Hirundinea contains the 500 or so species of animals commonly known as leeches. Except for the primitive Acanthobdella peledina leeches have no chaetae and 33 body segments. they have two suckers which in most cases are located one at the anterior (head) end of the body composed of segments 1-4 and the other at the posterior (tail) end composed of segments 25-33. Like the Oligochaeta from which they are believed to have evolved the Hirundinea occur in Fresh water, marine and terrestrial environments. (RW)

All annelids are characterized by the segementation of their body cavities into subdivisions that "partially transect the body cavity", which can also be referred to as metamerism. Segmentation is particularly useful for increasing the efficiency of locomotion by creating a system of muscle contractions whose effects are extremely localized. Furthermore, because each segment of the body cavity contains various aspects of body systems like the "circulatory, nervous, and extractory tracts," it allows for a larger level of intricacy in the organization of annelids' body plans. (MR; Source 20)

Annelids are both veriform and bilaterally symetrical. Being veriform means that they are have the long, thin, cylindrical shape of a worm, and their symmetry signifies that they are capable of being split into two equal parts so that one part is a mirror image of the other. Also, they have bodies with more than two cell layers, tissues and organs. (IL -source 8)

Their bodies can be divided into three regions: head, trunk and terminal region. The external wall of annelids in the trunk region is divided into a series of linear cylindrical segments, which can also be seen in the interior of the body. Therefore, every segment on the outer region of the body also corresponds to the internal division. (AM source 16)

The class Polychaeta is the most diverse of the three classes containing around 5,500 different types of species. For the most part they are marine animals. Polychaeta is further divided into 2 groups:
1). Sedentaria which live lethargic, dormant, and inactive lives in holes and
2). Errantia which live active lives swimming in open waters (AP- source 8)

Most Polychaetes are either male or female but some change sexes as they become more developed. During reproduction Polychaetes grow special reproductive segments which aid the release of gametes but these segments die/get destroyed after the process is over. (AP- source 8)
Evolved from marine animals that inhabited the bottom sand and mud of coastal waters, the Annelid phylum consists of three invertebrate classes that are most well known for their segmented structures. In an annelid, the body is divided into similar size segments that are known as metameres, where the youngest segments can be found at the rear of the annelid. All major organs such as the muscles, blood vessels, nerves, excretory organs, and reproductive organs repeat in each segment of the annelid. The digestive system however extends unbroken from mouth to anus. This metamerism or segmentation that the annelid displays is also seen in some chordates, a finding that shows convergent evolution in action. (ZJ- source 1)

Food and Digestion
Annelids mostly burrow for food, but some aquatic annelids will for food. The food
Pharynx, esophagus, crop, gizzard, and the intestine make up Annelids digestive system. The tract runs the length of the body.

external image digest.jpg(MB)
Annelida have a complete digestive system with mouth and anus. They have a crop, gizzard, and typhlosole, meant for storage, gridning and absorption, respectively. In oligochaetes, soil taken in might have high calcium levels, and so calciferous glands along the esophagus secrete calcium ions into the gut to decrease calcium ion concentration and maintain the acid-base balance. Polychaetes can have specialized feathery arms can make use of ciliary action and mucous as a trap to capture food particles. (JP- source 3)

After digestion, animals must excrete waste. The excretory system of the annelids has two metanephridia (a tube with cilia on one end of it and an excretory pore on the other end that opens to the outside of the body) per body segment. The function of these is to remove excess water in the body and to get rid of any nitrogen waste that is in the body. If an animal digests proteins, nitrogen must leave the body because it's toxic. Nitrogen can leave in as either ammonia, or uric acid. (MM)

One of the primary jobs of an earthworm is to maintain the soil in which they live. One way that this is carried out is through eating the dead leaves, roots, and other plant material in the soil called detritus.Worms that live in the top layer of the soil eat the freshly fallen leaves. Earthworms are important for agriculture because they prevent the soil from becoming too dense and allow the air to flow through the soil; this helps plant receive air and water from the surface to the roots in the soil. Often times, farmers and gardeners collect worms in the soil to maintain it along with fertilizer. (RK)

Circulatory System
Respiration is promoted by the circulatory system. Annelids have a closed circulatory system of vessels connected by a dorsal and ventral vessels. These vessels are connected by segmental pairs of vessels that run the length of the annelid. The vessels area pumping vessel.
The closed circulatory system of annelids means that blood flows entirely in closed vessels. Some species of annelids even have hearts. In addition, the blood of annelids contains hemoglobin, a protein which increases a blood cell's capacity to carry oxygen.
Annelida circulatory system (JP)
Annelida circulatory system (JP)

Annelids are hermaphrodites. Two of the same species of annelids orient themselves exchange sperm, the worms then separate. The sperm are stored while the clitellum, a special organ, secretes a mucous cocoon that moves through the annelids body picks up the eggs and then the sperm. The cocoon eventually slips off the worms head. The embryos then live in the soil. The Annelids can reproduce sexually or asexually. If they reproduce asexually they do it by fragmentation, budding, or fission. (MP)
Annelids are monoecious, organisms with both ovaries and testicles. Prior to copulation between two annelids, seminal fluid is passed from the testicles to the seminal vesicles, located between segments 9 and 12, as a storage area. During sex between two annelids, the two annelids line up in opposite directions. The sperm is releases from segment 15, onto the seminal receptacle located at segments 9 and 10. During mating both individuals deliver sperm to their partner. (MS 19)

Diagram of Annelid which especially highlights the reproductive organs (SP)
Diagram of Annelid which especially highlights the reproductive organs (SP)

The Phylum uses metanephrida, a tube in the annelids body, to excrete waste. This is the way annelids dispose of the metabolic waste.

In each segment of the annelid, there is a pair of excretory tubes called metaniphridia. A metaniphridium (the singular form) has a nephrostome on one end of it, and a nephridiopore on the excretory end, or where waste gets dumped. The nephrostomes push water, waste, and (unneeded) hormones into the metaniphridium. Capillaries surround the metaniphridium to reabsorb some water and substances. Eventually, the tube ends at a pore on the exterior of the annelid, where the waste is dumped in the form of urine. (MB) (Source #2)

the coelom separates annelids from other phylums. It is a opening in a organism that is surrounded by muscle that, in annelids, is right before the digestive tract. It protects the internal structures of an organism.
The coelom is a fluid-filled cavity that is between the gut and outer body wall of an annelid. It can also be used as a storage area and helps with the locomotion of an annelid. (CP- source 9)
The coelom is compartmentalized, with different sections separated by septa between segments. The compartments mean that if one or two segments is lost, the others maintain their contents, and can continue locomotion. (PS Source # 14)

Annelid (JS 22)
Annelid (JS 22)

Annelids use setae, a bristle-like structure, for moving around. The setae provide traction for burrowing. There are four pair of setae on each segment of the annelid. They use a longitudinal muscle and circular muscle to work a against the coelumatic fluid, a fluid found in the coelum, that works as a hydrostatic skeleton.
The animal has parapods, fleshy lobes supported by rods made of the carbohydrate, chitin. The parapods have setae, bristles, that can aid in locomotion. (AR)
Movement involves contracting and anchoring the setae to an object, like the ground or a rock. Muscles at the worm’s rear then contract, and the head moves forward. The reason this method of transportation works for annelids is that each muscle acts independently, although they all coordinate together during motion. When earthworms move, they aerate soil, helping plants grow and farmers to produce crops. (LPE)

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Osmotic Balance
Annelids are found in salt water, fresh water and under land. Salt water annelids most likely by drinking a lot of water. Fresh water annelids must excrete urine and eat in order to regulate their osmotic balance. Land annelids must maintain osmotic balance by eating and drinking regularly.

Phylum annelida seems to have no set type protection.Annelids have several forms of protection. Annelids that live in tunnels can easily withdraw into their homes when danger approaches and those that live near the surface move around quickly, jumping and thrashing widely. Sometimes an annelid may even purposely break off a few tail segments (which they can re-grow) to confuse a predator. Annelids also use camouflage to blend into their environments. Further, many annelids have tiny bristles on the ends of each of their segments that allow them to cling to the surface even as predators try to snatch them away (VN).Some species of annelids roll into balls and excrete mucous around themselves, forming a cocoon. This bodily state is called estivation. This function allows them to slow down wait for the soil conditions to improve. (AM source 17)
Anelidas usually have sharp setae that can be used for defense and protection. (AR)

Temperature Balance
Annelids match the ambient temperature, are ectotherms.
When annelids are not in favorable temperatures, they burrow deeper into the ground to match their body temperature. They do not have any internal mechanism to match the surrounding temperature (no thermoregulation), so to maintain homeostasis, they have to physically move. (SP)

1. Barnes, Robert.
Invertebrate Zoology. 2nd. Philadelphia, PA: W.B. Saunders Company, 1969. Print.



4. (TB)

5. <a href="">Earthworms: Oligochaeta - Behavior And Reproduction</a> (VN)

6. (MB)

7. (AR)

8. "The Annelids (Phylum Annelida)." The Earth Life Web. Web. 29 Oct. 2011. (AP)

9. "Characteristics of Annelida."
Tree of Life Web Project. Web. 29 Oct. 2011. <>. (CP)
10. (MP)

11. Raw, F. "Studies of Earthworm Populations in Orchards - RAW - 2008 - Annals of Applied Biology."
Wiley Online Library. Web. 30 Oct. 2011. <>. (SP)

12. "Flatworms, Mollusks, Annelids." 2 Nov. 2011 (MM)

13. "Biological Diversity 8."
Estrella Mountain Community College. Web. 06 Nov. 2011. <>. (SP)


15. (JP- picture) (LPE)
16. "Annelida Characteristics" Web. 31 Oct. 2011. <>
17. Conrad, Jim. "Earthworms."
Backyard Nature with Jim Conrad.
Web. 25 Oct. 2011. <

18. Earthworks: Oligochaeta <>
Kingdom Animalia PHYLUM ANNELIDA//. Web. 13 Nov. 2011.

20. Myers, P. 2001. "Annelida" (On-line), Animal Diversity Web. Accessed November 13, 2011 (MR)

21. (RW)
22. (JS)