In addition to bundles of chaetae, noto- and neuropodia can also have a variety of cirri and gills.
General features
They are most elaborate in actively crawling or swimming forms where they form large fleshy lobes that act as paddles. Parapodia of burrowing or tubicolous polychaetes can simply be slightly raised ridges carrying hooked chaetae called uncini. Annelids have a brain or cerebral ganglion that originates and usually resides in the head. The brain varies in structure, with mobile active forms having the most complex brains, and sessile or burrowing forms having simple brains with little differentiation. It is connected to the ventral nerve cord by the circumpharyngeal connectives, which run down each side of the pharynx.
The ventral nerve cord, is usually made up of a pair of cords that are bound together and runs the length of the body Fig. It varies in thickness and dilates into a ganglion in each segment, from which pairs of segmental nerves pass out to the body wall, muscles and gut.
Ventral view of a scaleworm Sigalionidae showing its double nerve cord. This worm is unusual in having red pigmentation associated with the cord, making it easily seen. There are six major kinds of sensory structures found in annelids. These include palps, antennae, eyes, statocysts, nuchal organs and lateral organs Fig. Palps and antennae are located on the head of many polychaetes.
In some groups they are both sensory while in others the palps are used for feeding. Nuchal organs are ciliated, paired, chemosensory structures, innervated from the posterior part of the brain. They are present in nearly all polychaetes, and Rouse and Fauchald suggested that they may represent an apomorphy for Polychaeta. Annelids also have a variety of epidermal sensory cells that may be responsive to light or touch such as lateral organs. A closed circulatory system is present in most polychaetes Fig. It is reduced or absent in leeches where it may be replaced by coelomic canals.
Annelid Reproduction: Facts & Example
A limited circulatory system in which some of the major blood vessels are present but the distal capillary vessels are missing is found in a number of polychaete groups. A circulatory system is absent in many small polychaetes. In most annelids there are usually two fluid systems, the coelom and the circulatory system, and both if present are involved in the excretion of waste products. Sinauer Associates, Sunderland, MA. Help us improve the site by taking our survey.
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Classification Kingdom Animalia animals Animalia: Related Taxa Species Hirudo medicinalis Hirudo medicinalis: The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility , annelids serve humans as food and as bait. Scientists observe annelids to monitor the quality of marine and fresh water.
Although blood-letting is used less frequently by doctors, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength.
Since annelids are soft-bodied , their fossils are rare — mostly jaws and the mineralized tubes that some of the species secreted. There are over 22, living annelid species, [5] [6] ranging in size from microscopic to the Australian giant Gippsland earthworm and Amynthas mekongianus Cognetti, , which can both grow up to 3 metres 9. The Archiannelida , minute annelids that live in the spaces between grains of marine sediment , were treated as a separate class because of their simple body structure, but are now regarded as polychaetes.
No single feature distinguishes Annelids from other invertebrate phyla, but they have a distinctive combination of features. Their bodies are long, with segments that are divided externally by shallow ring-like constrictions called annuli and internally by septa "partitions" at the same points, although in some species the septa are incomplete and in a few cases missing.
Most of the segments contain the same sets of organs , although sharing a common gut , circulatory system and nervous system makes them inter-dependent. However, the frontmost and rearmost sections are not regarded as true segments as they do not contain the standard sets of organs and do not develop in the same way as the true segments. The segments develop one at a time from a growth zone just ahead of the pygidium, so that an annelid's youngest segment is just in front of the growth zone while the peristomium is the oldest.
This pattern is called teloblastic growth. The phylum's name is derived from the Latin word annelus , meaning "little ring". Annelids' cuticles are made of collagen fibers, usually in layers that spiral in alternating directions so that the fibers cross each other. These are secreted by the one-cell deep epidermis outermost skin layer. A few marine annelids that live in tubes lack cuticles, but their tubes have a similar structure, and mucus -secreting glands in the epidermis protect their skins.
Below this are two layers of muscles, which develop from the lining of the coelom body cavity: The setae "hairs" of annelids project out from the epidermis to provide traction and other capabilities. The simplest are unjointed and form paired bundles near the top and bottom of each side of each segment. The chetoblasts produce chetae by forming microvilli , fine hair-like extensions that increase the area available for secreting the cheta. When the cheta is complete, the microvilli withdraw into the chetoblast, leaving parallel tunnels that run almost the full length of the cheta.
Nearly all polychaetes have parapodia that function as limbs, while other major annelid groups lack them. Parapodia are unjointed paired extensions of the body wall, and their muscles are derived from the circular muscles of the body.
They are often supported internally by one or more large, thick chetae. The parapodia of burrowing and tube-dwelling polychaetes are often just ridges whose tips bear hooked chetae. In active crawlers and swimmers the parapodia are often divided into large upper and lower paddles on a very short trunk, and the paddles are generally fringed with chetae and sometimes with cirri fused bundles of cilia and gills.
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The brain generally forms a ring round the pharynx throat , consisting of a pair of ganglia local control centers above and in front of the pharynx, linked by nerve cords either side of the pharynx to another pair of ganglia just below and behind it. From each segmental ganglion a branching system of local nerves runs into the body wall and then encircles the body. As in arthropods , each muscle fiber cell is controlled by more than one neuron , and the speed and power of the fiber's contractions depends on the combined effects of all its neurons.
Vertebrates have a different system, in which one neuron controls a group of muscle fibers. Their large diameter decreases their resistance, which allows them to transmit signals exceptionally fast.
Distribution and abundance
This enables these worms to withdraw rapidly from danger by shortening their bodies. Experiments have shown that cutting the giant axons prevents this escape response but does not affect normal movement. The sensors are primarily single cells that detect light, chemicals, pressure waves and contact, and are present on the head, appendages if any and other parts of the body.
Most annelids have a pair of coelomata body cavities in each segment, separated from other segments by septa and from each other by vertical mesenteries. Each septum forms a sandwich with connective tissue in the middle and mesothelium membrane that serves as a lining from the preceding and following segments on either side. Each mesentery is similar except that the mesothelium is the lining of each of the pair of coelomata, and the blood vessels and, in polychaetes, the main nerve cords are embedded in it.
Parts of the mesothelium, especially on the outside of the gut, may also form chloragogen cells that perform similar functions to the livers of vertebrates: Many annelids move by peristalsis waves of contraction and expansion that sweep along the body , [7] or flex the body while using parapodia to crawl or swim. The fluid in the coelomata contains coelomocyte cells that defend the animals against parasites and infections.
In some species coelomocytes may also contain a respiratory pigment — red hemoglobin in some species, green chlorocruorin in others dissolved in the plasma [20] — and provide oxygen transport within their segments. Respiratory pigment is also dissolved in the blood plasma. Species with well-developed septa generally also have blood vessels running all long their bodies above and below the gut, the upper one carrying blood forwards while the lower one carries it backwards. Networks of capillaries in the body wall and around the gut transfer blood between the main blood vessels and to parts of the segment that need oxygen and nutrients.
Annelid Reproduction: Facts & Example | theranchhands.com
Both of the major vessels, especially the upper one, can pump blood by contracting. In some annelids the forward end of the upper blood vessel is enlarged with muscles to form a heart, while in the forward ends of many earthworms some of the vessels that connect the upper and lower main vessels function as hearts. Species with poorly developed or no septa generally have no blood vessels and rely on the circulation within the coelom for delivering nutrients and oxygen. However, leeches and their closest relatives have a body structure that is very uniform within the group but significantly different from that of other annelids, including other members of the Clitellata.
They function as the main blood vessels, although they are side-by-side rather than upper and lower. However, they are lined with mesothelium, like the coelomata and unlike the blood vessels of other annelids. Leeches generally use suckers at their front and rear ends to move like inchworms. The anus is on the upper surface of the pygidium. In some annelids, including earthworms , all respiration is via the skin.
However, many polychaetes and some clitellates the group to which earthworms belong have gills associated with most segments, often as extensions of the parapodia in polychaetes. The gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow. Feeding structures in the mouth region vary widely, and have little correlation with the animals' diets. Many polychaetes have a muscular pharynx that can be everted turned inside out to extend it.
In these animals the foremost few segments often lack septa so that, when the muscles in these segments contract, the sharp increase in fluid pressure from all these segments everts the pharynx very quickly. Two families , the Eunicidae and Phyllodocidae , have evolved jaws, which can be used for seizing prey, biting off pieces of vegetation, or grasping dead and decaying matter. On the other hand, some predatory polychaetes have neither jaws nor eversible pharynges. Selective deposit feeders generally live in tubes on the sea-floor and use palps to find food particles in the sediment and then wipe them into their mouths.
Filter feeders use "crowns" of palps covered in cilia that wash food particles towards their mouths. Non-selective deposit feeders ingest soil or marine sediments via mouths that are generally unspecialized. Some clitellates have sticky pads in the roofs of their mouths, and some of these can evert the pads to capture prey. Leeches often have an eversible proboscis, or a muscular pharynx with two or three teeth. The gut is generally an almost straight tube supported by the mesenteries vertical partitions within segments , and ends with the anus on the underside of the pygidium.
The bacteria convert inorganic matter — such as hydrogen sulfide and carbon dioxide from hydrothermal vents , or methane from seeps — to organic matter that feeds themselves and their hosts, while the worms extend their palps into the gas flows to absorb the gases needed by the bacteria.
Annelids with blood vessels use metanephridia to remove soluble waste products, while those without use protonephridia.