Annelida means ‘little ring’ or also called segmented worms. Segmentation or also can be known as metamerism is the repetition of body parts along the length of the body. It is also segmented both internally and externally. The segments of annelids are separated internally by septa as shown in Figure 3 and externally by annuli which is a shallow ring-like constrictions (Mader, 2010). Each segment has a ganglion, locomotory structure and a pair of excretory organs. That means each segments contain same organs . A well-developed coelom is a fluid-filled between the outer body wall and the gut as referred to Figure 4. Not all annelids have it but most of it have coelom. Coelom also acts as a supportive hydrostatic skeleton for locomotion. Along with partitioning of the coelom, a hydrostatic skeleton allows independent movement of each body segment. Because of that, annelids can crawl on the surface instead of just burrowing in the mud (Mader, 2010). Intersegmental septa divided the coelom as a series of compartments. That means, if the worm is damaged, the locomotion can be maintained because the coelomic contents will only be lost from a few segments.The phylum Annelida traditionally can be divided into three main groups, Polychaeta (the polychaetes), Oligochaeta (the Oligochaetes) and Hirudinea (the leeches). But, a phylogenomic study in 2011 and other recent studies show that oligochaetes are a subgroup of the polychaetes making the polychaetes a paraphyletic group. Hence, the traditional names are no longer can be used to describe the evolutionary history of annelids. Current evidence indicates that the annelids can be divided into two major clades, Errantia and Sedentaria. Errantia is a large and diverse group whose most of the members are marine such as Nareimyra punctata. Meanwhile, Sedentaria tends to be less mobile than Errantia such as Spirobranchus giganteus.Annelids can be monoecious with permanent gonads or dioecious with temporary or seasonal gonads. Digestive wastes of annelids pass out through the anus. Meanwhile, cellular waste containing nitrogen is eliminated by nephridia, excretory organs that filter fluid into the coelom. There is also a pair of excretory tubules called metanephridia in each segment of annelids as shown in Figure 5. It has tubules that will produce dilute urine for excretion.Clitellata annelids are hermaphrodites which have both testes and ovaries. But, it needs two annelids to mate and reproduce. There is a reproductive organ in the clitellum as shown in Figure 6. Two clitellata annelids will lie parallel to each other facing in the opposite direction when they are mate. They are held in place by mucus secreted by the clitellum. The clitellum then will form a cocoon which protects the developing eggs .Clitellata annelids do not have lungs. They breathe through their skin. Oxygen and carbon dioxide pass through the skin by diffusion. For diffusion to occur, the skin needs to keep moist always. Because of that, they usually at the surface of the soil at night when it is possibly cooler and in the soil at daylight because they can be affected by the sun. Clitellata annelids have only a few satae which are chitinous hair-like structures normally only eight small setae per body segment. Setae are arranged in pairs which protrude directly from the surface of the Clitellata annelids have a round body with tapered end.Annelids under order opisthopora have closed circulatory blood system. Blood flows through vessels and not direct contact with the organs.The plane can divide the body into half. Each half is similar to one another.Most of the annelids use their pharynx just like in Figure 8 to get good. Some of the annelids have two sharp jaws to attack prey. While some of their pharynx are covered in mucus to feed on decaying vegetation. Not just that, other annelids filter feed, they fan water through tube and catch food particles in a mucous bag .This mean that the blood is contained within hearts and blood vessels. In the dorsal blood vessel, the red blood moves anteriorly, which connect to ventral blood vessel by five pairs of muscular vessel called “hearts” as shown in Figure 9. Hemoglobin are also present in red blood cell.They live in moist and damp soil to keep the wall moist for gas exchange and if they are too exposed to sunlight, they will die.The basic nervous system is a single or double nerve cord running along the ventral side of the body with an enlarged region (ganglion) in each segment. For example earthworm as shown in Figure 10 have larger ganglia in the head region that serve as a simple “brain” .The size differs from species to species and individual to an individual in the same species. The size of a mature earthworm can be up to 150 mm in length and 3-5 in width.Slimy to touch and glistening dark-brown in colour due to the presence of the pigment porphyrin in its body wall in order to protect from the effect of the bright light.The body wall of Pheretima is very thin, soft, shiny and elastic which consists of cuticle and epidermis.In the coelom on the sides of the dorsal vessel, there is a small paired glandular structures from 26th segments up to the last. They also contain phagocytes which ingest foreign materials and digest them.With the help of three types of sense organs which are receptor epidermal receptors, buccal receptors and photoreceptors, Pheretima can reacts to a number of external stimuli .The body wall consists of cuticle layer, an epidermis, a circular muscle layer, a longitudinal muscle layer and a peritoneum from outer to inner layer.
Clitellum does not have in younger earthworm. Its tranverse section displayed mucous cocoon and albumin secreting glands. The function of mucous is to maintain earthworm position when mating.The range of body size is 45-700 mm.Accomplished section by section, uses muscle contraction and the setae. Segments bulge and their setae protrude into the soil when longitudinal muscle contract, then, the setae are withdrawn when circular muscles contract and these segments move forward.
One of the advantages of Pheretima sp. or also can be known as earthworm is they can improve soil structure. This is because they produces feces in the form of cast which will cement the soil particles together. Thus, the soil structure will improve and it will help to hold the water and nutrient without dispersing them. Next, earthworm also can be used as a bait by fishermen. As all know, fishes feed on worms, because of that, fishermen used earthworms as a bait. Not only that, earthworms also help to improve soil drainage. Because of the extensive network of burrows and channels they create in the soil, indirectly, they loosen up the soil and improve its drainage. Nowadays, earthworms are used in the making of compost manure known as vermicompost. Vermiculture is the cultivating of earthworms mainly for the purpose of using them to make vermicompost from organic waste.Earthworms also have their own disadvantages. First, they are invasive species. At least, fifteen non-native terrestrial species of earthworms have been introduced in U.S. Not just that, earthworms also can damage the forests. In the situation without worms, leaves that will fall to forest floor will decompose slowly and create spongy layer of organic material that provides a natural growing environment for native wildflowers. That leaves also help to prevent soil erosion. If earthworms are presence, they can turn over 5 tons of organic matter a year. Next, earthworms also will upset soil chemistry. A huge amount of Pheretima sp. will release excessive amount of nitrogen into the soil and it may harm other living things. Earthworms also make it difficult for flowers to thrive. This is because Pheretima sp. can disrupt the root systems of native woodland wildflowers and reduce the diversity of plants. Lastly, they can disrupt golf courses and sport fields such as create muddy messes on the green