The annelids include earthworms, polychaete worms, and leeches. All members of the group are to some extent segmented, in other words, made up of segments that are formed by subdivisions that partially transect the body cavity. Segmentation is also called metamerism. Segments each contain elements of such body systems as circulatory, nervous, and excretory tracts.
Metamerism increases the efficiency of body movement by allowing the effect of muscle contraction to be extremely localized, and it makes possible the development of greater complexity in general body organization. Besides being segmented, the body wall of annelids is characterized by being made up of both circular and longitudinal muscle fibers surrounded by a moist, acellular cuticle that is secreted by an epidermal epithelium.
All annelids except leeches also have chitonous hair-like structures, called setae, projecting from their cuticle. Sometimes the setae are located on paddle-like appendages called parapodia.
Annelids are schizocoelous and with a large and well-developed true coelom i. Except in leeches, the coelom is partially subdivided by septa. Hydrostatic pressure is maintained across segments and helps maintain body rigidity, allowing muscle contractions to bend the body without collapsing it. The internal organs of annelids are well developed.
They include a closed, segmentally-arranged circulatory system. The digestive system is a complete tube with mouth and anus. Structures such as the excretory, locomotory and respiratory organs are generally repeated in each segment.
Segments are formed sequentially in annelids and are established during development from growth zones located at the posterior end of the body; so the youngest segment in the body of an annelid is always the most posterior. The only parts of the annelid body that are not segmental are the head and a terminal post-segmental region called the pygidium.
The head is comprised of two units, the prostomium and the peristomium. The postsegmental pygidium includes the zone from which new segments are proliferated during growth.
The proposed homology of segmentation seen in annelids with that seen in Arthropoda has been used to unite the two as Articulata, a grouping that dates back to Cuvier The homology of this segmentation has been questioned recently, with arthropods now viewed by many as closer to taxa such as Nematoda Aguinaldo et al. This suggests that the form of segmentation seen in annelids may in fact represent an apomorphy.
With regards to the supposedly unsegmented Echiura, their reinstatement within Annelida see McHugh suggests that their apparently unsegmented body in fact represents a series of fused segments see Hessling and Westheide Figure 2. Ophryotrocha Dorvilleidae.
Sandgerdi Iceland. A distinctive feature of annelids are structures called chaetae Fig. Chaetae also called setae are bundles of chitinous, thin-walled cylinders held together by sclerotinized protein. They are produced by a microvillar border of certain invaginated epidermal cells and so can be defined as cuticular structures that develop within epidermal follicles.
Chaetae show a huge amount of variation, from long thin filaments capillary chaetae to stout multi-pronged hooks Fig. Apart from annelids, chaetae are found in Echiura and Brachiopoda. There is now good evidence Hessling and Westheide, ; McHugh, that the former group falls within Annelida.
There is a distinct possibility therefore that chaetae represent an apomorphy for Annelida. Figure 3. Proscoloplos Orbiniidae. Bondi, Australia. SEM and Light micrographs. The most recent comprehensive systematization of polychaetes, that proposed by Rouse and Fauchald from their cladistic parsimony analyses, has been used here Fig. Allowing for the likely errors in the placement of many taxa, and the fact that there were conflicting results included in the original analyses, the most fundamental problem inherent in the systematization used here may be that of the placement of the root for any tree of Annelida.
Their trees also excludes Echiura from Annelida. This result was based on outgroup choices such as Mollusca and Sipuncula, and may well be misleading. Alternative hypotheses are therefore worth outlining, though they do not follow normal cladistic practice.
Storch , following a detailed study on the musculature of Annelida, proposed that scale-worms, a diverse clade within Phyllodocida, are representative of the plesiomorphic condition for Annelida. He suggested that there was a radiation from this group, but that Chrysopetalidae were most closely related to scale-worms. The implication of his hypothesis is that Phyllodocida represents a paraphyletic group, from which all other polychaete taxa arise.
Westheide and see Westheide et al. This would either result in a paraphyletic Phyllodocida or Amphinomida, depending on which taxon is used as the root Fig. The conflict between this molecular sequence data and the morphological results could be caused by several factors. Further morphological study, combined with sequence data, may uncover these 'losses' see Hessling and Westheide, However, the molecular sequence data sets assembled to date have been marked by both a limited number of taxa and characters.
An exception is Brown et al. They recovered some morphological groupings such as Cirratulidae, Terebellidae and Eunicida, but did not show a monophyletic Phyllodocida or Aciculata, nor did they find any parts of these taxa to be basal groups of Annelida. However, some expected groupings were not recovered.
None of the major taxa used here, such as Palpata, Aciculata, Phyllodocida, Canalipalpata, Sabellida or Terebellida were recovered in Martin's analysis. Also less diverse taxa such as Nereididae, Spionidae and Aphroditiformia were not recovered. In a review of the fossil record of annelids Rouse and Pleijel suggested that the oldest unequivocal fossil polychaetes, such as Canadia from the Cambrian, belong within Phyllodocida. Subsequent fossil polychaetes that can be confidently placed outside Phyllodocida do not appear until the Carboniferous.
No other fossil polychaetes from the Cambrian can be unequivocally assigned to extant polychaete taxa. There are several likely appearances from the Ordovician, including Serpulidae, Spionidae and the radiation of Eunicida.
Ensuing appearances suggest that by the end of the Carboniferous most major polychaetes lineages had appeared. The exception appears to be Scolecida, with the earliest known fossils being the dubious Archarenicola Arenicolidae from the Triassic, and one assignable to Paraonidae from the Cretaceous.
With the rooting option employed in Figure 1, it appears that some of the earliest appearing fossil polychaetes belong to derived clades e.
This could be interpreted in two ways: 1 the root placement in Figure 1 is wrong, and so Aciculata, comprised of Amphinomida, Eunicida and Phyllodocida, may in fact represent a paraphyletic 'stem' group for the rest of polychaetes; 2 a number of major polychaete clades had already evolved in, or before, the 'Cambrian explosion', but fossils have not yet been found.
The third possibility is that the overall topology used in Figure 1 may be profoundly incorrect. If we accept that the basic topology shown in Figure 1 is correct, but do not root the tree, then a diagram as shown in Figure 5 is the result. This may represent the most conservative representation of our understanding of annelid relationships. Aguinaldo, A. Turbeville, L. Linford, M. Rivera, J. Garey, R. Raff, and J. Evidence for a clade of nematodes, arthropods and other moulting animals. Nature Bartolomaeus, T.
Structure and formation of the uncini in Pectinaria koreni, Pectinaria auricoma Terebellida and Spirorbis spirorbis Sabellida : implications for annelid phylogeny and the position of the Pogonophora. Brown, S. Rouse, P. Hutchings, and D. Cuvier, G. Deterville, Paris. Eibye-Jacobsen, D. A new genus and species of Dorvilleidae Annelida, Polychaeta from Bermuda, with a phylogenetic analysis of Dorvilleidae, Iphitimidae and Dinophilidae. All the animals in the clitellata class have a clitellum.
This fat band is the reproductive area of the animal. Clitellates the name for worms with a clitellata nurse their eggs in the clitellum before excreting them into the environment. They secrete these eggs as cocoons. Since annelids have no body skeleton, few parts of their bodies can be fossilized.
Neither the oligochaeta nor leeches have any hard body parts, meaning we have next to no fossil records of these animals. Polychaeta, on the other hand, sometimes have hard jaws and some species create mineralized tubes. Both these structures can be fossilized. The earliest fossil records we have of annelids are from million years ago. Half a billion years ago, life underwent the Cambrian Explosion. Life rapidly diversified during this era. Animals were just beginning to emerge from the ocean onto land.
The fossil record seems to suggest that annelids benefitted from the Cambrian Explosion like many other phyla. While we often think of large vertebrates and trees when we think of ecosystems, the small animals beneath our feet can be just as important.
Annelids, whether they are forest floor decomposers or ocean predators, are an integral part of the web of life. Annelid worms have many other significant impacts on our planet than are covered here.
The following are just a few examples of how these worms impact our planet. Earthworms oligochaetes are vital parts of most soil systems. These worms eat decaying organic matter and decompose it in their gut. Their excrement is nutritious for both plant and soil microbe communities.
These plant and microbe communities can more easily use the nutrients in the worm feces than in an entire leaf. Worms speed up the process of decomposition. In addition to speeding up decomposition, earthworms increase the ability of soil to catch and retain water.
As they move through the topsoil, worms leave small tunnels of air behind them. These tunnels act as pipes into the soil for water to drain into. Moreover, the decomposed worm feces is much better at retaining moisture than undecomposed organic matter. Therefore, earthworms invite more water into the soil and help that water stay there longer.
The process of earthworms making these porous soil tunnels has another beneficial impact on the soil. As they wriggle up and down, earthworms slowly till the soil. The physical act of moving the soil is one way they till the soil. They also eat matter on the surface and excrete it well below the surface and vice-versa.
All this movement means that earthworms can effectively till the top six inches of soil in as little as ten years! Based on biomass , worms are the most abundant invertebrates in the soil!
We should all thank earthworms for the invaluable services they provide our food and ecosystems. Red wrigglers eat an eggshell in a vermicomposting environment. Some people have harnessed the decomposing power of earthworms to make ultra-fertile compost.
Vermiculture is the use of worms to break down food waste into usable compost for plants. People cultivate certain species of worms for the purpose of vermiculture. The most commonly used species is Eisenia fetida , or the red wriggler. The African and European nightcrawlers E. These worms can break down all sorts of tough food, such as egg shells and bones. Many sustainability-focused home gardeners practice vermiculture because the worms cut down on food waste and create compost with comparable or better quality than that of store-bought compost.
If you are interested in vermiculture, all you need is a pound of red wrigglers, a dark container, some saw dust, and some food waste. It really is as simple as that! People also practice vermiculture for composting toilets. These worms will eat human poop and decompose it from hazardous biowaste into nutrients available for plants.
This process is largely fragrance-free because earthworms secrete anti-bacterial chemicals from their coelom when they poop. These anti-bacterial chemicals prevent the smelly odors that bacteria emit from plaguing the composting toilet.
Along with beavers, elephants, corals, and other important species, earthworms are ecosystem engineers. This means that they have an outsized impact on their ecosystems, often by modifying the ecosystem extensively. The course of evolution of life on Earth can be attributed, in some part, to annelids. Before the Cambrian Explosion, organisms were mostly simple and lived in the oceans. A major turning point in global evolution was the increase of carbon dioxide in the atmosphere, which led to global heating in the Cambrian era.
New research suggests that the evolution of animals that burrow on the ocean floor caused this global heating. Many of these burrowing animals were likely prehistoric annelids. Before this burrowing, the ocean floor was a solid algal mat where little biogeochemical cycling occurred.
Burrowers poked innumerous holes in this algal mat, introducing oxygenated ocean water to previously anaerobic substrate. As this matter decomposed, it released vast amounts of carbon into the atmosphere while consuming oxygen in the ocean.
Over time, this bacteria used up enough of the oxygen in the ocean to cause an extinction event. Those worms might look small, but their collective impact is profound. Soil is both a major source and sink of greenhouse gases. Surprisingly, earthworms disappeared altogether in much of North America during the previous Ice Age, beginning about 12, years ago.
As glacial ice sheets moved south through the continent, no space was left for earthworms. As a result, the modern forests of the northern U.
Instead of worm decomposers, fungus and other soil invertebrates broke down the organic matter in northern North American forests. The introduction of these earthworms has significantly changed the ecology of these forests.
These non-native earthworms can significantly increase the greenhouse gas emissions from forest soils. Soil accounts for about 20 percent of global greenhouse gas emissions.
Earthworms are expected to rapidly increase their range throughout Canada in the coming decades. This increased decomposition could have a significant impact on climate change. Annelids are one of the favorite baits used by fisherpeople around the world. The same species used for vermicomposting, the red wrigglers, are coveted for fishing bait because their wriggling behavior attracts fish. Interestingly, bait worms are many times more expensive , per pound, than lobster!
Each year, humans use over , tonnes of bait worms for fishing. A study revealed that the bait worm industry is worth over six billion dollars. These worms are usually collected by hand, with people gathering worms in mud flats during low tide. A close-up of a colony of giant tubeworms. Photo by National Oceanic and Atmospheric Administration. These bizarre creatures are certainly one of the most extreme annelids. They are marine worms that live exclusively on hot hydrothermal vents on the ocean floor of the eastern Pacific.
They can be found at a depth of 2 miles below the ocean surface! The giant tubeworms take the cake for the heaviest annelids and can grow over six feet tall. The anatomy of giant tubeworms is different from most annelids. Instead of digesting food with a gut like normal worms, these unusual beings use bacteria to digest sulfur.
Sulfur naturally flows out of the hydrothermal vents on the ocean floor. Since they are stationary, giant tubeworms build hard tubes around their soft bodies to protect them from predators. They can retract their feathery plume when prospective predators get too close. The feathery plume is the organ that exchanges gasses between the worm and the ocean environment.
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