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The Diagnostic characteristics that define Annelida:
The main way that Annelida, segmented worms, are recognized is by their segmented bodies. They look like little rings put together which is actually the definition of Annelida, “little ring.” Annelids have a three-region body consisting of the head, many segments, and the pygidium. The head is located at the anterior end of the body, and is made up of two components, the prostomium, and the peristomonium. The second part of the body, the one that takes up most of the body cavity, is the segments, which are recurring pieces. Each segment resembles a ring, and has a coelom, which is a fluid-filled area. Additionally, each segment contains its own locomotory, respiratory, and excretory structures. The last body region, the pygidium, is located at the posterior end of the body, and is composed of an area in which new segments are created throughout development (AK). The worms range in shape, size, and color they can be as tiny as 1mm to as large as 3m. They live in mostly the sea and in damp soil. A common Annelida that many people know is the earthworm.
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( MSR)

There are three different Classes to the Annelida:
Class Oligochaeta:

This class includes the earth worm and different kinds of aquatic worms. There are about 3,000 known species. They mostly eat detritus and algae. Oligochates are most common in freshwater biomes. They can tolerate low dissolved oxygen levels and can survive in organically polluted areas. (LD) Aquatic oligochaetes are eaten by fish and larger vertebrates. [Like the polychaetes, oligochaetes have bodies divided into segments. However, they lack parapodia and, with a few exceptions, have relatively few and inconspicuous setae. The setae are usually arranged in four bundles on each segment; those of aquatic forms are longer than those of land forms. The setae of an earthworm may be felt as a roughness if one rubs a finger along its side. RS]

Class Polychaeta:
Polychaeta means many setae, they have bristles on each segment and they have a pair of paddle like structures, which are called parapodia. Worms in this class include fan worms and many other marine worms. They live mostly in water, in the plankton, and some dig themselves in the sand and shells. The majority of annelids are polychaetes; there are about 9,000 species of polychaetes and they can be found in almost every marine habitat on earth. (APS)
Fan worms
Fan worms

Class Hirudinea:
This class includes leeches which mostly live in fresh water and are invertebrate, blood suckers. Leeches can actually be used for medical reasons to help with blood flow. They usually live in warm, still waters and prefer to stay out of the light. (JAS) [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.RS]

Acquiring and digesting food:
Worms get there food by going through the ground or in the sea they take in the food through their mouth than it runs through there digestive system and leaves through there annus. There digestive system is made up of several different regions the pharnx, a muscular tube that starts at the ventral side and ends at the mouth, the esophagus, a channel that moves food from the pharynx to the stomach, the Crop, the Gizzard, the Intestine. At each segment of the worm has an excretion tubes that allow the worm to excrete waste these tubes are called metanephridia. These Metanephridia have funnels in them that are called nephrostomes that remove waste from the blood stream and coelomic fluid. The whole digestive tract is lined with blood vessels and nerve cords which penetrate the septa, which is a divider in-between cells that allow small particles to pass through its pores.
added by AS.
added by AS.

Sensing the environment:
The Annelida have a pair of cerebral ganglia which are a cluster of nerve cells in the central nervous system that are located above and in front of the Pharynx. Nerves run around the pharynx and connect to a sub pharyngeal ganglion, and then from their nerves go posteriorly, outward. All through these nerve cords are segmental ganglia, which are just separated packs of nerves, which in this case are fused. These bunches of cords help the worm sense the environment and over all feel and function. With the exception of leeches, all annelids have setae (as previously mentioned), which are sharp, spine-like hairs extending from the body. In addition to being used for locomotion, the setae are used for sensing changes in the environment [KNS].
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The Cerebral Ganglia of the  Annelid. (Walker K.)
The Cerebral Ganglia of the Annelid. (Walker K.)

The worm uses setae, which are bristles that provide traction and allow the worm to borrow in its environment. Setae are composed of chitinous cylinders and develop within a worm's epidermal follicles. They can vary from long, thin filaments (known as capillary setae) to short multi-pronged hooks (JM). Many Annelida use a set of two muscles to move around. The first is the longitudinal and the second is the circular muscle. These muscles work in the hydrostatic skeleton, fluid which is held under pressure in a closed body compartment, to make the worm move. Movement occurs by extending the body, anchoring it to a surface by using the setae, and then contracting the body muscles. When moving forward, the circular muscles at the anterior end contract, causing a wavelike contraction originating in the circulatory muscles to pass toward the posterior end. When the wave of contraction nears the mid-section of the body, the longitudinal muscles contract, shortening the region. A wave of contraction of longitudinal muscles follows, and the cycle is repeated (LS).

Respiration/ Circulation:
The Annelida has a closed circulatory system that is made up of webs of veins and five pairs of heart-like structures (AZ) that carry oxygen enriched blood. Dorsal and ventral vessels are connected by other vessels that surround the esophagus that are muscular and enable the worm to pump blood through its system. Tiny blood vessels are all throughout the worms skin which works as the worm’s respiratory system.

Waste Removal System:
The metanephridia go to the outside of the worm and allow the worm to excrete metabolic waste and discharge. The nephridia, which act much like our kidneys do by removing waste products from the blood (MSR), are repeated in each segment of the worm. (AZ) The worm also uses its anus to excrete waste.

Self Protection:
The worm burrows itself underground which protects the worm from many predators. But another way some, but not all, Annelida protect themselves is that they can be cut in half and re-grow from those two halves, this is also one way the worm can reproduce asexually.

Reproduction is either asexual (by itself) or sexual. Asexual reproduction is either from fragmentation, budding, or fission. Sexually reproducing species have external eggs, most terrestrial species encase the eggs in cocoons.(MSR)

Review Questions:
1. Define setae and explain its importance to organisms of Annelida. (JAF)
2. Describe how annelida acquire and digest their food. (JJF)
3. Do annelida have an open or closed circulatory system? How does it work. Explain how they perform respiration. (DG)
4. Although they appear to be simple organisms, annelida have a complex anatomical structure. What is the significance of the annelida's segments, which comprise a majority of the organism? [Ben Yudysky]
5. How many classes of annelida are there, what are they, and what is the most common habitat for each? (Jesse Landy)

Sources: (Ali Kirsch) (Katey Duchin) (Sara Waugh) (Rabya Saraf) (Jackson Murphy) (Jake Schwartz) Schwartz) (Alyssa Zisk) rubin) (Sarah Fleming) (Walker K.) (Liz Daley) (Katie Shikora)[Rabya S] McDermott)