Body plan of earthworms, as shown by Lumbricus terrestris, is extraordinarily constant and will be utilized to show body plan of annelids. Some of the differences which happen between the species and members of other classes will be mentioned along the way. Body plan is best shown in transverse section and three-dimension.
Lumbricus terrestris a representative annelid:
Lumbricus terrestris is one of the more common annelids, happening often in pasture and woodland soils that are not also acid or too waterlogged. It lies inside the burrow which may be up to 2 meters deep at times of cold or drought; burrows often end in a small chamber. Walls of burrows are prevented from collapsing by the cement lining of defaecated soil. Adult worm can reach maximum length of 300 mm. Organization of earthworms and other annelids are generally explained as tube within a tube. In transverse section, anterior third of its body is round, but posterior two-thirds of body shows some degree of dorso-ventral flattening. Body is composed of approx 150 segments, with conspicuous annuli marking positions of internal septa. Prostomium is decreased to a small cone, and peristomium to simple segment with ventral mouth. Chaetae, which are composed principally of polysaccharide chitin held together with scleroprotein fibres, are extremely short, and project only slightly out of skin. Each chaeta is secreted by the single cell that lies at base of chaetal sac. Protractor and retractor muscles related with chaetae enable them to be protracted or withdrawn. Clitellum is conspicuous structure, mainly in sexually mature worms. It engages segments 32 to 36 or 37, and is formed by epidermal glandular thickening.
Structure of body wall is very similar to that of Nereis. Thin cuticle, comprising mainly of collagen fibres with polysaccharide and gelatin, is secreted by epidermis that is simple columnar epithelium with many mucus-secreting glands and sensory cells. Mucus secreted by epidermis passes onto surface of worm through perforations in cuticle. Sensory cells comprise touch and chemical receptors that are randomly distributed through epidermis; photoreceptors are also present. There is also nerve net just below epidermis. Body wall is completed by well-developed layer of circular muscle and layer of longitudinal muscle that is arranged in nine blocks. These muscle blocks contain very elaborate structure, and are strengthened by lamellae of collagen, that are well supplied with blood vessels. Muscle fibre is ribbon-like and made up of single elongated cell; cells are grouped in bundles and arranged along sides of lamellae.
Features of oligochaetes, the coelom is entirely subdivided by well-developed septa (singular: septum) with many bundles of radial, circular, and oblique muscle fibres. Some communication between segments is possible by sphinctered aperture lying immediately dorsal to nerve cord, though this is usually closed. In each segment, sphinctered mid-dorsal pore opens from coelom by body wall to exterior. Coelomic fluid can be exuded through this pore onto skin, to give extra surface moisture.
Digestive system and feeding:
Lumbricus feeds on decaying fallen leaves together with saprophytic fungi and bacteria that cause their decay. It gathers these from soil surface at night and pulls them in burrow, while it stays anchored to burrow by tip of posterior end of body. Large quantities of soil are swallowed in feeding and also during burrowing. Gut is simple. Mouth opens in a thin-walled buccal cavity that is slightly thickened dorsally to form the tongue. Pharynx that opens from buccal cavity, is principal swallowing organ. It is thick-walled muscular structure, and contains both intrinsic muscles, and radiating extrinsic radial muscles that are inserted onto body wall. Contractions of the muscles pump food in mouth. Pharynx opens in oesophagus, that is differentiated in 3 regions: thin walled tube extending to segment, a crop, and gizzard. Crop is thin walled and used as storage organ. Calciferous glands are features of oligochaete oesophagus, and are associated to its burrowing habit. They are lateral evaginations of oesophageal wall that have no direct digestive function, but serve to remove excess calcium and carbonates taken in with soil that could unbalance pH of body fluids. Excess calcium and carbonate ions are gathered in calciferous glands creating calcite crystals which then leave with faeces. The long, straight intestine completes gut. Enzymes generated within intestine of Lumbricus comprise cellulase and chitinase, so animal can digest cell walls of plants and exoskeleton' of soil arthropods. Intestine has the inner layer of circular muscle fibres and outer layer of longitudinal muscle fibres. Chloragogenous cells are not limited to Lumbricus, but happen in several oligochaetes and polychaetes in different regions comprising oesophagus, nephridia, intestine, and principal blood vessels.
Whether crawling or burrowing, worm proceeds by extension, anchoring, and contraction rather than by undulation. Every segment becomes alternately short and fat (by contraction of longitudinal muscle) or long and thin (by contraction of circular muscle). Alternating waves of contraction pass along body from anterior to posterior, the new wave starting as first passes in posterior half. Chaetae point in posterior direction, to permit segments move forwards effortlessly, but preventing them from sliding back. Segments in the state of longitudinal contraction protract the chaetae to anchor them to ground; segments in which circular muscle is contracted withdraw the chaetae and are either pulled up or pushed forward.
Blood circulatory system and exchange of gases:
Blood is similar in composition to that of Nereis, and has respiratory pigment hemoglobin in solution. Circulatory system is the closed system and based on similar plan, involving the combination of longitudinal and segmental blood vessels. Main collecting vessel is dorsal blood vessel that runs for most of its length in close contact with gut. The ventral blood vessel is suspended in mesentery ventral to gut, and in addition, 3 significant longitudinal vessels, median sub-neural vessel and paired, lateral neural vessels are found in association with nerve cord. Dorsal vessel receives blood from body wall using segmental parietals, and from gut via 3 vessels per intestinal segment from typhlosole and two from gut plexus capillaries. In all segments, except anterior 11 it is connected to sub-neural vessel by pair of dorsosubneural vessels that run in septa. Ventral blood vessel is principal route of distribution, giving off 5 main vessels in every segment. The pair of ventro-parietals extends to body wall, and 3 ventro-intestinals to gut plexus. Nephridia and reproductive organs are supplied with blood by small branches from ventro-intestinal vessels.
Annelids have two types of respiration depending on different classes. If annelids live in sea or water like annelids in class Polychaeta, they contain gills to breathe. Few annelids don't have lugs like earthworm (class Oligochaeta), and they utilize their outer skin to exchange gas. They get oxygen from soil and release carbon dioxide from skins. After they obtain oxygen, they exchange gas in body through circulatory system. So their skins require keeping wet to make gas exchange possibly. Respiration of annelids in class Hirudinea like leeches is like earthworm. They also breathe by their skins. Though, leeches live in water not in soil. If some leeches can't get sufficient oxygen in water, they will move to surface.
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