Annelida Characteristics

Phylum annelida is distributed all across the world, on land as well as in water. Earthworms, leeches are some popular members of this phylum. Other annelida examples include all sorts of worms that we are familiar with. Although, some species of this phylum are aquatic, majority of the species are terrestrial. Annelids possess some peculiar characteristics which are unique to their phylum.

Annelids Characteristics

Anatomy
Annelids are creatures with perfectly bilateral symmetry. The body can be divided into three regions: head, trunk and terminal. The external wall of annelids in the trunk region is divided into a series of linear cylindrical segments. This division pattern is also followed in the interior of the body. Thus, every segment on the outer region of the body also corresponds to the internal division.

Habitat
Annelids can survive in a range of geographical conditions, from semi-arid regions to heavy rainfall regions. 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.

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 annelids characteristic. It is responsible for the rigidity of the animal and also serves as skeleton. It acts like a muscular support during locomotion and the proper functioning of the annelid is dependent upon the volume of coelomic fluid.

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.

Respiration
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.

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.

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.

Besides, there are also several other important characteristics of annelids like ability to regrow most of the lost body parts, which sets them apart.