|Classes and subclasses|
*Some authors consider the subclasses
The annelids (from Latin anellus "little ring") are a large phylum (Annelida) of invertebrate animals, comprising the segmented worms, including the well-known earthworms and leeches. There are about 15,000 known modern species of annelids.
Annelids are found in most wet environments and include many terrestrial, freshwater, and especially marine species (such as the polychaetes), as well as some which are parasitic or mutualistic. They range in length from under a millimeter to over three meters (the seep tube worm Lamellibrachia luymesi).
In addition to enhancing the diversity of nature, which brings so much joy to humans, the segmented worms are ecologically and medically important. They are common in marine, freshwater, and terrestrial environments, serving as part of the food chain, and helping to turn over the soil and sediments. Some segmented worms have commercial use as bait for sport fishing or food for tropical aquarium fish. The leech can be used medically, for example, in controlling swelling, as it produces chemicals that can serve as an anesthetic and prevent blood coagulation.
The three major groups of annelids are the polychaetes (largely marine annelids, with over 5,500 species); the oligochaetes (earthworms and freshwater worms, with over 3,000 species); and the hirundinea (leeches, with about 500 species). However, biological classification of annelids can vary widely among taxonomists.
Some consider there to be three classes of annelids: Polychaeta, Clitellata, and Aelosomata. The Clitellata are then further divided into three or four subclasses: Oligochaeta (earthworms and freshwater worms), Hirundinea (leeches), and Branchiobdella (about 150 species of small animals that are largely parasites or commensals on crayfish), and sometimes Acanthobdellida (leech-like, temporary parasite, which is also placed in Hirundinea in some classifications). Aelosomata includes small to very small annelides, with about 25 known species. This taxonomy looks like the following:
Another taxonomic scheme regards two groups of polychaetes—the Archiannelida and the Myzostomaria—as classes in their own right, and recognizes four total classes: Polychaeta, Clitellata, Myzostomida, and Archiannelida. This looks like the following:
In some biological classifications, the Clitellata is considered a subphylum and the Oligochaeta, Hirudinea, and Branchiobdellida are treated as classes of this subphylusm.
A simple classification scheme is to recognize two classes of annelids, the Polychaeta and the Clitellata:
There have also been proposals to consider the Clitellata as part of the Polychaeta, thus making the latter term synonymous with the annelids.
Annelids are triploblastic protostomes with a coelom (at least historically), closed circulatory system, and true segmentation. Protosomes are animals with bilaterial symmetry where the first opening in development, the blastophore, becomes its mouth. Triploblastic means that they have three primary tissue areas formed during embryogenesis. A coelom is a fluid-filled body cavity.
Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is largely filled in with tissue and reduced to a system of narrow canals; archiannelids may lack the coelom entirely. The coelom is divided into a sequence of compartments by walls called septa. In the most general forms, each compartment corresponds to a single segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. Each segment is marked externally by one or more rings, called annuli. Each segment also has an outer layer of circular muscle underneath a thin cuticle and epidermis, and a system of longitudinal muscles. In earthworms, the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of muscles between the outer circulars and inner longitudinals. In most forms, they also carry a varying number of bristles, called setae, and among the polychaetes a pair of appendages, called parapodia.
Anterior to the true segments lies the prostomium and peristomium, which carries the mouth, and posterior to them lies the pygidium, where the anus is located. The digestive tract is quite variable but is usually specialized. For example, in some groups (notably most earthworms) it has a typhlosole (internal fold of the intestine or intestine inner wall), to increase surface area, along much of its length.
Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders that simply ingest the sediments, and blood-suckers.
The vascular system and the nervous system are separate from the digestive tract. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel that conveys the blood in the opposite direction. The two systems are connected by a vascular sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body wall.
The nervous system has a solid, ventral nerve cord from which lateral nerves arise in each segment. Every segment has an autonomy; however, they unite to perform as a single body for functions such as locomotion. Growth in many groups occurs by replication of individual segmental units; in others, the number of segments is fixed in early development.
Asexual reproduction by fission is a method used by some annelids and allows them to reproduce quickly. The posterior part of the body breaks off and forms a new individual. The position of the break is usually determined by an epidermal growth. Lumbriculus and Aulophorus, for example, are known to reproduce by the body breaking into such fragments. Many other taxa (such as most earthworms) cannot reproduce this way, though they have varying abilities to regrow amputated segments.
Sexual reproduction allows a species to better adapt to its environment. Some annelida species are hermaphroditic, while others have distinct sexes.
Most polychaete worms have separate males and females and external fertilization. The earliest larval stage, which is lost in some groups, is a ciliated trochophore, similar to those found in other phyla. The animal then begins to develop its segments, one after another, until it reaches its adult size.
Earthworms and other oligochaetes, as well as the leeches, are hermaphroditic and mate periodically throughout the year in favored environmental conditions. They mate by copulation. Two worms, which are attracted by each other's secretions, lay their bodies together with their heads pointing in opposite directions. The fluid is transferred from the male pore to the other worm. Different methods of sperm transference have been observed in different genera, and may involve internal spermathecae (sperm storing chambers) or spermatophores that are attached to the outside of the other worm's body. The clitellata lack the free-living ciliated trochophore larvae present in the polychaetes, the embryonic worms developing in a fluid-filled "cocoon" secreted by the clitellum.
The annelid fossil record is sparse, but a few definite forms are known as early as the Cambrian. There are some signs they may have been around in the later Precambrian. Because the creatures have soft bodies, fossilization is an especially rare event. The best-preserved and oldest annelid fossils come from Cambrian Lagerstätten, such as the Burgess Shale of Canada, and the Middle Cambrian strata of the House Range in Utah. The annelids are also diversely represented in the Pennsylvanian-age Mazon Creek fauna of Illinois.
The arthropods and their kin have long been considered the closest relatives of the annelids on account of their common segmented structure. However, a number of differences between the two groups suggest this may be convergent evolution rather than a feature passed on by common descent.
The other major phylum that is of definite relation to the annelids is the mollusk, which shares with them the presence of trochophore larvae. Annelids and Mollusks are thus united as the Trochozoa, a taxon more strongly supported by molecular evidence.
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