Everything about Annelida totally explained
The
annelids, collectively called
Annelida (from
Latin anellus "little ring"), are a large
phylum of
animals comprising the segmented
worms, with about 15,000 modern species including the well-known
earthworms and
leeches. They 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 3 meters (the seep tube worm
Lamellibrachia luymesi).
Physiology
Annelids are
bilaterally symmetric and
triploblastic protostomes with a
coelom (which makes them coelomates), closed circulatory system and true segmentation. Their segmented bodies and coelom have given them evolutionary advantages over other worms.
Oligochaetes and
polychaetes typically have spacious
coeloms; in leeches, the coelom is 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 triple segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. The closed circulatory system consists of networks of vessels containing blood with oxygen-carrying
hemoglobin. Dorsal and ventral vessels are connected by segmental pairs of vessels. The dorsal vessel and five pairs of vessels that circle the esophagus of an earthworm are muscular and pump blood through the circulatory system. Tiny blood vessels are abundant in the earthworm's skin, which function as its respiratory organ. Each segment (metamere) 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 and in daria 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 (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 which simply ingest the sediments, and blood-suckers. Annelids can also grow up to six inches.
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 which 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
nerve cord from which lateral
nerves come in contact with 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.
Depending upon the species, annelids can reproduce both sexually and asexually.
Asexual reproduction
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 identical worm. The position of the break is usually determined by an epidermal growth.
Lumbriculus and
Aulophorus, for example, are known to reproduce by the penis breaking into such fragments. This complete
regeneration is noteworthy as these Annelid species are the most highly organized animals to have this capability. Many other taxa (such as most earthworms) can't reproduce this way, though they've varying abilities to regrow
amputated segments.
Sexual reproduction
Sexual reproduction allows a species to better adapt to its environment. Some annelida species are
hermaphroditic, while others have distinct sexes.
Most polychaete worms are gonochoristic, that is, they've 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 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 clitella lack the free-living ciliated trochophore larvae present in the polychaetes, the embryonic worms developing in a fluid-filled "cocoon" secreted by the
clitellum.
Fossil record
The annelid
fossil record is sparse, but a few definite forms are known as early as the
Cambrian, and there are some signs they were around in the earlier
Precambrian, but the earliest unequivocal annelid fossils are only known from the former. Because the creatures have soft bodies, fossilization of a body is an especially rare event. However, a few annelids, such as the living polychaetes in the
Serpulidae, secrete calcareous tubes, and such tubes are fairly common as fossils (although these are not necessarily from annelida, as other animal phyla can also secrete tubes). The hard jaws of certain polychaetes, known as
scolecodonts, are known from the
Ordovician onward, and are common enough to be used for
stratigraphic correlation in some cases. The best-preserved and oldest annelid body fossils come from the 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. A few small groups have been treated as separate phyla: the Pogonophora and Vestimentifera, now included in the family
Siboglinidae, and the
Echiura.
Relationships
The
arthropods and their kin have long been considered the closest relatives of the annelids, on account of their common segmented structure, giving rise to the grouping of
Articulata. However, a number of differences between the two groups suggest this may be
convergent evolution. The other major phylum which is of definite relation to the annelids are the
molluscs, which share with them the presence of
trochophore larvae. Annelids and Molluscs are thus united as the
Trochozoa, a taxon more strongly supported by molecular evidence.
Sipuncula,
Echiura and
Siboglinidae have traditionally been placed in their own phyla, while Clitellata has been considered separated from the polychaete annelids. But recent research indicates that all of them actually belongs within the
Polychaete, even if some of these groups have lost their segmentation
(External Link
).
Classes and subclasses of Annelida
- Clitellata
- Oligochaeta - The class Oligochaeta includes the megadriles (earthworms), which are both aquatic and terrestrial, and the microdrile families such as tubificids, which include many marine members as well.
- Leeches (Hirudinea) - These include both bloodsucking external parasites and predators of small invertebrates.
- Aphanoneura
- Polychaeta - This is the largest group of annelids and the majority are marine. All segments are identical each with a pair of parapodia. The parapodia are used for swimming, burrowing and the creation of a feeding current.
Further Information
Get more info on 'Annelida'.
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