The annelids, collectively called Annelida, 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, as well as some which are parasitic or mutualistic. They range in length from under a millimetre to over 3 metres.
Annelids are triploblastic protostomes. The body cavity is a coelom, a fluid-filled cavity in which the gut and other organs are suspended. 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 coleom 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 usually specialized. 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.
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 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.
Depending upon species, annelids can reproduce both sexually and asexually.
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 can regrow the posteriormost segments in most instances.
Sexual reproduction allows a species to better adapt to its environment. Some annelid species are hermaphroditic, while others have distinct genders.
Hermaphrodite annelids like earthworms mate periodically throughout the year in favored environmental conditions. Earthworms 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 tranference 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.
Most polychaete worms have separate males and females and external fertilization. The earliest larval stage, which is lost in some groups, is a ciliated trocophore, similar to those found in other phyla. The animal then begins to develop its segments, one after another, until it reaches its adult size. The oligochaetes and leeches tend to be hermaphroditic and lack free_living larvae of this sort. While annelids have some regenerative abilities, sometimes to the point where each half of an adult divided cross_wise will survive, this is not universal, and especially does not occur among the earthworms as folklore would suggest.
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 later Precambrian. A few small groups have been treated as separate phyla: the Pogonophora and Vestimentifera, now included in the family Siboglinidae, and the Echiura.
The arthropods and their kin have long been considered the closest relatives of the annelids, on account of their common segmented structure, but 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. These groups are united as the Trochozoa, and when the arthropods are included, they and the annelids are treated in a subgroup called the Articulata.
Classes and subclasses of Annelida
- 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.
- Polychaeta - This is the largest group of the Annelids and majority are marine. All segments are identical each consisting a parapodia. The parapodia are used for swimming, burrowing and feeding current.