Phylum Annelida is distributed all across the world, on land as well as in water. Earthworms and 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, a majority are terrestrial. Annelids possess some peculiar characteristics which are unique to their phylum. These characteristics are listed below.
Annelids are creatures with a perfectly bilateral symmetry. Their body can be divided into three regions, the head, trunk, and terminal. The external wall 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.
Annelids can survive in a range of geographical conditions, from semi-arid regions to heavy rainfall regions. However, most annelids prefer moist, damp, or waterlogged areas for shelter. They can also be found under rocks or logs, which provide ideal hiding places.
The digestive system of annelids is in the form of an unsegmented gut which runs along the length of the body. It starts from the mouth and ends at the anus, located in the terminal region. The gut is separated from the body wall with a fluid-filled body cavity called the coelom. The coelom is a very unique characteristic of these creatures. It is responsible for the rigidity of the animal and also serves as the skeleton. It acts as a muscular support during locomotion and the proper functioning of the annelid is dependent upon the volume of coelomic fluid.
The nervous system is primitive and underdeveloped in annelids. The brain is in the form of a ganglionic mass and is located in the head region. 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.
Some aquatic annelids have thin-walled gills as respiratory organs. Gases are exchanged between the 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 the body wall itself.
Reproduction takes place either sexually or asexually. Fission, budding, and fragmentation are the common mediums of asexual reproduction. 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.
The entire body of annelids is covered with small hairlike structures called setae or chaetae. These structures are composed of chitin and help the annelid to grip the surface properly. The segmented body wall is equipped with ring muscles which help in locomotion. In aquatic annelids, the setae are used to increase the surface area of the appendages to facilitate swimming.
Besides, there are also several other important characteristics of annelids like the ability to regrow most of their lost body parts, which sets them apart.