Annelids are a large Phylum of Segmented Worms, with over 17,000 modern Species including Ragworms, Earthworms and Leeches. Annelids are considered members of the Lophotrochozoa, a 'Super-Phylum' of Protostomes that also includes Molluscs, Brachiopods, Flatworms and Nemerteans. Most Annelids are free-living, although several Species have parasitic, mutualistic, or commensal relationships with other Animals, and many Species are commonly associated with aquatic and terrestrial Plants. 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.

• Scientific Classification

• Types

• Characteristics of Annelids
  
  • Digestive System
  • Circulatory System

• History & Evolution
  

• Annelids Reproduction

In addition, the Pogonophora, Echiura and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are soft-bodied, their fossils are rare. Annelids like Earthworms are exceedingly important in soil creation, particularly in temperate areas. They keep soils aerated, and their castings fertilize the soil. Soils may harbour 50 to 500 Earthworms per square meter which is good for agriculture. In marine environments many Species of Polychaetes play a fundamentally important role in the maintenance of food chains and the whole ecological balance of the seas, thus supporting the stocks of Fish which Humans consume.

The 2000 IUCN Redlist includes:

Critically Endangered Species

1. Mesonerilla prospera
2. Phallodrilus macmasterae

Vulnerable Species

1. Washington Giant Earthworm - Driloleirus americanus
2. Oregon Giant Earthworm - Driloleirus macelfreshi
3. Earthworm - Komarekiona eatoni
4. Giant Gippsland Earthworm - Megascolides australis

Another 145 Species of Annelids including the Medicinal Leech 'Hirudo medicinalis' and the Palolo Worm 'Eunice viridis' are also suspected of being in need of conservation, however too little is actually know about their populations for them to be included in the IUCN lists.

Annelids Scientific Classification

Kingdom - Animalia.
Superphylum - Lophotrochozoa.
Phylum - Annelida.

Types: Annelids

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Classes and Subclasses of Annelids

Polychaetes - There are about 12,000 Species.

Clitellates - There are about 5,000 Species.

Clitellates Suborder

• Oligochaetes.
• Hirudinea.

Characteristics of Annelids

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• Metamerism (segmentation)- Annelids have segmented bodies. They are divided into 3 segments. These segments are also called Metameres. Their bodies are long, with segments that are divided externally by shallow ring-like constrictions called Annuli and internally by 'Septa' (partitions) at the same points, although in some Species the Septa are incomplete and in a few cases missing. Each segment is, in principle, limited by Septa dividing it from neighbouring segments, and has a fluid-filled cavity within referred to as a Coelom. The Coelom is fluid-filled, which creates hydrostatic (water) pressure and acts as a Hydrostatic Skeleton. Most of the segments contain the same sets of organs, although sharing a common gut, circulatory system and nervous system makes them inter-dependent.

3 SEGMENTS:

1.  Prostomium - It is the front most section and contains the brain and sense organs. The first section behind the Prostomium is called the Peristomium. The head is comprised of the Prostomium and the Peristomium.

2.  Trunk - The external wall of Annelids in the trunk region is divided into a series of linear cylindrical segments.

3.  Pygidium - Pygidium is the rearmost section and is the only part of the Annelid body that is not segmental. It is a terminal post-segmental region containing the anus, generally on the underside. Sometimes the Pygidium is also referred as a tail, though it is not actually a tail.

Segments are formed sequentially in Annelids and are established during development from growth zones located at the posterior end of the body; so the youngest segment in the body of an Annelid is always the most posterior. The number of segments is relatively fixed in some groups (Branchiobdellida, Hirudinea), but indeterminate in others.

• Cuticle Based Outer Covering - Annelids have a body covered by an 'External Cuticle' that is never shed or moulted. It does not contain cells but is secreted by cells in the skin underneath. Epidermal microvilli secrete a network of fibres that are in part collagenous and also contain scleroprotein. Chaetae are also cuticular structures, but contain large amounts of chitin. Beneath the epidermis and its basal lamina lies a layer of circular muscle. The circular muscle layer forms a nearly continuous sheath around the body, except in polychaetes with well-developed parapodia (appendages). Beneath the circular muscle layer lie thick longitudinal muscles. In many Annelids the longitudinal muscles are present as four distinct bands.

• Bilaterally Symmetrical - It is the balanced distribution of duplicate body parts or shapes.

• Closed Circulatory System - Annelids have a well-developed, 'Closed Circulatory System' (one in which blood is limited to vessels) that is segmentally arranged.

• Annelid Respiratory System - Some Aquatic Annelids have thin walled gills as respiratory organs. Gases are exchanged between blood and the environment through these gills. However in Terrestrial Annelids, there is an absence of distinct respiratory organs. The exchange of gases takes place through the medium of body wall itself.

• Annelids Locomotion - The entire body of Annelids is covered with small hair like structures called Setae or Chaetae. These structures are composed of Chitin and help the Annelid to grip the surface properly. The segmented body wall of the Annelids is equipped with ring muscles which help in locomotion. In aquatic Annelids, the Setae are used to increase the surface area of appendages to facilitate swimming.

• Annelids Digestive System - Annelida digestive system is in the form of an unsegmented gut which runs along the length of the body. It starts from the mouth and ends at anus, located in the terminal region. The gut is separated from the body wall with a fluid filled body cavity called Coelom. Coelom is a very unique Annelid feature. It serves as skeleton for Annelids and provides rigidness to the body. It acts like a muscular support during locomotion.

• Annelids Nervous System - The nervous system is primitive and underdeveloped in Annelids. The brain is in the form of ganglionic mass and is located in the head region on Annelids. It is connected to the ventral nerve cord through a network of ring nerves. The ventral nerve cord has branches which lead to lateral nerves and ganglia in each segment.

• Annelid Sensory System - The sensors are primarily single cells that detect light, chemicals, pressure waves and contact, and are present on the head, appendages (if any) and other parts of the body. Nuchal (on the neck) organs are paired, ciliated structures found only in Polychaetes, and are thought to be Chemosensors. Some Polychaetes also have various combinations of Ocelli (little eyes) that detect the direction from which light is coming and camera eyes or compound eyes that can probably form images. The compound eyes probably evolved independently of arthropods' eyes. Some Tube-Worms use Ocelli widely spread over their bodies to detect the shadows of Fish, so that they can quickly withdraw into their tubes. Some burrowing and tube-dwelling Polychaetes have Statocysts (tilt and balance sensors) that tell them which way is down. A few Polychaete genera have on the undersides of their heads Palps that are used both in feeding and as 'Feelers', and some of these also have antennae that are structurally similar but probably are used mainly as 'Feelers'.

• Regeneration - Most Polychaetes and Oligochaetes also use similar mechanisms to regenerate after suffering damage. Two Polychaete genera, Chaetopterus and Dodecaceria, can regenerate from a single segment, and others can regenerate even if their heads are removed.  Annelids are the most complex Animals that can regenerate after such severe damage. On the other hand leeches cannot regenerate.

Weight - The 'Largest Annelid' can weigh over 1.5 kg (3.1 lb) and the 'Smallest Annelid' weighs about few milligrams.

Size - The 'Largest Annelid' measures between 3 meters to 6.7 meters (9.8 -22 feet) in length and the 'Smallest Annelid' measures 0.5 mm (0.02 ins) in length.

Geographical Range and Habitat

They are found worldwide from the deepest marine sediments to the soils in our city parks and yards. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments. . However, most Annelids prefer moist, damp or water logged areas for shelter. They can also be found under the rocks or logs, which provide ideal hiding places for Annelids.

Diet - Some Annelids are hunters, the prey usually being other small Invertebrates. Many Annelids are Detritivores, feeding on sediments and deposits for what they contain in terms of smaller Invertebrates and micro-organisms. Ragworms and Terrestrial Earthworms feed in this manner. Some Leeches such as the well known Medicinal Leech are blood feeders on Vertebrates.

Predators - They are a major part of the diet for many Fish, Moles, Hedgehogs, Black Bird etc. A variety of other Birds, Mammals, Reptiles, other Annelids etc. are Predators of Annelids.

History and Evolution of Annelids

The fossils of Annelids are rare as they are soft bodies Animals. 'Phragmochaeta' from Sirius Passet, is considered to be about 518 million years old, making it the oldest Annelid known to date as per Simon Conway Morris and John Peel's findings. Polychaetes diversified in the early Ordovician, about 488 to 474 million years ago. The Annelids are considered to have evolved in the sea, perhaps from an ancestral Flatworm that evolved through the Trochophore Larva, the characteristic early stage of Polychaetes. The Oligochaetes are thought to have developed from Polychaete stock; the Leeches, which have the clitellum in common with the Oligochaetes, probably evolved from the latter.

Annelids Reproduction

Reproduction in Annelids takes place either sexually or asexually. Fission, budding and fragmentation are the common mediums of asexual production. In some classes, hermaphrodite sexuality is observed, while marine Annelids reproduce by producing free swimming larvae from fertilized eggs. Eggs of terrestrial Annelids are enclosed inside the cocoons which eventually develop into miniature versions of the adults. Most Earthworms and Leeches are Hermaphroditic with both Male and Female gonads (glands). Polychaetes usually have separate sexes; many Polychaetes hatch into a particular type of Planktonic Larva, the Trochophore, which later metamorphoses into a juvenile Annelid. They sink to the sea-floor and metamorphose into miniature adults: the part of the Trochophore between the Apical Tuft and the Prototroch becomes the Prostomium (head); a small area round the Trochophore's anus becomes the Pygidium (tail-piece); a narrow band immediately in front of that becomes the growth zone that produces new segments; and the rest of the Trochophore becomes the Peristomium (the segment that contains the mouth).Some Polychaetes, however, can reproduce asexually, by budding. About 14% of Annelids use a external fertilization but produce yolk-rich eggs, which reduce the time the larva needs to spend among the Plankton, or eggs from which miniature adults emerge rather than larvae. The rest care for the fertilized eggs until they hatch - some by producing jelly-covered masses of eggs which they tend, some by attaching the eggs to their bodies and a few Species by keeping the eggs within their bodies until they hatch. These Species use a variety of methods for sperm transfer; for example, in some the Females collect sperm released into the water, while in others the Males have penis  that inject sperm into the Female.