Annelid

• Class Polychaeta (paraphyletic?)
• Class Clitellata*
  • Oligochaeta - Earthworms and others
  • Acanthobdellida
  • Branchiobdellida
  • Hirudinea - Leeches
• Class Myzostomida
• Class Archiannelida (polyphyletic)
• Class Echiura

*Some authors consider the subclasses
under Clitellata to be classes

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.

Anatomy

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 septae. 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 consists of a dorsal vessel conveying the blood, and a ventral longitudinal vessel which conveys the blood in the opposite direction.

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 occurs by replication of individual segmental units.

Reproduction

Depending upon species, annelids can reproduce both sexually and asexually.

Asexual reproduction

Asexual reproduction enables annelids to regenerate lost parts. 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 are known to reproduce by the body breaking into such fragments.

Sexual reproduction

Annelids that reproduce sexually are more adapted to the environment. Some species are hermaphroditic, while others have distinct genders.

Hermaphrodite annelids like earthworms mate periodically throughout the year in the favored environmental conditions. Earthworms mate by the method of copulation. Two worms which are attracted by each other's secretions lie their bodies together with their heads pointing opposite directions. The fluid is transfered from the male pore into the other worm's body. Different methods of sperm tranference have been observed in different genera that help them reproduce more effectively in their given environments.

Most polychaete worms have separate males and females. 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.

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 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.

Relationships

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

• 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.

• 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.