phylum annelida

Phylum Annelida

The annelids (also called "ringed worms"), formally called Annelida (from Latin anellus "little ring"), are a large phylum of segmented worms, with over 22,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments.They are bilaterally symmetrical, triploblastic, coelomate organisms.They have parapodia for locomotion. Although most textbooks still use the traditional division into polychaetes (almost all marine), oligochaetes (which include earthworms) and leech-like species, research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora, Echiura and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a "super-phylum" of protostomes that also includes molluscs, brachiopods, flatworms and nemerteans.

The basic annelid form consists of multiple segments, each of which has the same sets of organs and, in most polychaetes, a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly-defined or absent in some, and Echiura and Sipuncula show no obvious signs of segmentation. In species with well-developed septa, the blood circulates entirely within blood vessels, and the vessels in segments near the front ends of these species are often built up with muscles to act as hearts. The septa of these species also enable them to change the shapes of individual segments, which facilitates movement by peristalsis ("ripples" that pass along the body) or by undulations that improve the effectiveness of the parapodia. In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, and there is a wide range of locomotory techniques – some burrowing species turn their pharynges inside out to drag themselves through the sediment.

Although many species can reproduce asexually and use similar mechanisms to regenerate after severe injuries, sexual reproduction is the normal method in species whose reproduction has been studied. The minority of living polychaetes whose reproduction and lifecycles are known produce trochophore larvae, which live as plankton and then sink and metamorphose into miniature adults. Oligochaetes are full hermaphrodites and produce a ring-like cocoon round their bodies, in which the eggs and hatchlings are nourished until they are ready to emerge.

Earthworms support terrestrial food chains both as prey and by aerating and enriching soil. 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 no longer in favor with 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. Although some late Ediacaran fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about 518 million years ago in the early Cambrian period. Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous, about 299 million years ago. Scientists disagree about whether some body fossils from the mid Ordovician, about 472 to 461 million years ago, are the remains of oligochaetes, and the earliest certain fossils of the group appear in the Tertiary period, which began 65 million years ago.

Class of annelida:

1. Polychaeta

The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000 species are described in this class. Common representatives include the lugworm (Arenicola marina) and the sandworm or clam worm Nereis.

Polychaetes as a class are robust and widespread, with species that live in the coldest ocean temperatures of the abyssal plain, to forms which tolerate the extreme high temperatures near hydrothermal vents. Polychaetes occur throughout the Earth's oceans at all depths, from forms that live as plankton near the surface, to a 2–3 cm specimen (still unclassified) observed by the robot ocean probe Nereus at the bottom of the Challenger Deep, the deepest spot in the Earth's oceans. Only 168 species (less than 2% of all polychaetes) are known from freshwaters.

Polychaetes are segmented worms, generally less than 10 centimetres (3.9 in) in length, although ranging at the extremes from 1 millimetre (0.039 in) to 3 metres (9.8 ft). They are often brightly coloured, and may be iridescent or even luminescent. Each segment bears a pair of paddle-like and highly vascularized parapodia, which are used for movement and, in many species, act as the worm's primary respiratory surfaces. Bundles of bristles, called setae, project from the parapodia.

However, polychaetes vary widely from this generalised pattern, and can display a range of different body forms. The most generalised polychaetes are those that crawl along the bottom, but others have adapted to many different ecological niches, including burrowing, swimming, pelagic life, tube-dwelling or boring, commensalism, and parasitism, requiring various modifications to their body structure.

The head, or prostomium, is relatively well developed, compared with other annelids. It projects forward over the mouth, which therefore lies on the animal's underside. The head normally includes two to four pair of eyes, although there are some blind species. These are typically fairly simple structures, capable of distinguishing only light and dark, although some species have large eyes with lenses that may be capable of more sophisticated vision.

The head also includes a pair of antennae, tentacle-like palps, and a pair of pits lined with cilia, known as "nuchal organs". These latter appear to be chemoreceptors, and help the worm to seek out food.

2. Clitellates

Clitellates (about 10,000 species). These have few or no chetae per segment, and no nuchal organs or parapodia. However, they have a unique reproductive organ, the ring-shaped clitellum ("pack saddle") round their bodies, which produces a cocoon that stores and nourishes fertilized eggs until they hatch or, in moniligastrids, yolky eggs that provide nutrition for the embyros. The clitellates are sub-divided into:

A.  Oligochaeta

Oligochaeta is a subclass of animals in the biological phylum Annelida, which is made up of many types of aquatic and terrestrial worms, and this includes all of the various earthworms. Specifically, it contains the terrestrial megadrile earthworms (some of which are semi- or fully aquatic), and freshwater or semi-terrestrial microdrile forms including the tubificids, pot worms and ice worms (Enchytraeidae), blackworms (Lumbriculidae) and several interstitial marine worms.

With around 10,000 known species the Oligochaeta make up about one half of the phylum Annelida. These worms usually have few setae (chaetae) or "bristles" on the outer body surface, and lack parapodia, unlike polychaeta.

Oligochaetes are well-segmented worms and most have a spacious body cavity (coelom) that is used as a hydroskeleton. They range in length from less than 0.5 millimetres (0.020 in) up to 2 to 3 metres (6.6 to 9.8 ft) in the 'giant' species such as the giant Gippsland earthworm and the Mekong Worm Amynthas mekongianus (Cognetti, 1922).

The first segment, or prostomium, of oligochaetes is usually a smooth lobe or cone without sensory organs, although it is sometimes extended to form a tentacle. The remaining segments have no appendages, but they do have a small number of bristles, or chaetae. These tend to be longer in aquatic forms than in the burrowing earthworms, and can have a variety of shapes.

Each segment has four bundles of chaetae, with two on the underside, and the others on the sides. The bundles can contain anything from one to twenty-five chaetae, and include muscles to pull them in and out of the body. This enables the worm to gain a grip on the soil or mud as it burrows into the substrate. When burrowing, the body moves peristaltically, alternately contracting and stretching to push itself forward.

A number of segments in the forward part of the body are modified by the presence of numerous secretory glands. Together, they form the clitellum, which is important in reproduction.

B.   Hirudinea 

Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like other oligochaetes, such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from other oligochaetes in significant ways. For example, leeches do not have bristles and the external segmentation of their bodies does not correspond with the internal segmentation of their organs. Their bodies are much more solid as the spaces in their coelom are dense with connective tissues. They also have two suckers, one at each end.

The majority of leeches live in freshwater environments, while some species can be found in terrestrial and marine environments, as well. Most leeches are hematophagous, as they are predominantly blood suckers that feed on blood from vertebrate and invertebrate animals. Almost 700 species of leeches are currently recognized, of which some 100 are marine, 90 terrestrial and the remainder freshwater taxa.

Leeches, such as the Hirudo medicinalis, have been historically used in medicine to remove blood from patients. The practice of leeching can be traced to ancient India and Greece, and continued well into the 18th and 19th centuries in both Europe and North America. In modern times, the practice of leeching is much rarer and has been replaced by other contemporary uses of leeches, such as the reattachment of body parts and reconstructive and plastic surgeries and, in Germany, treating osteoarthritis.