Annelida are also known as the segmented animals. They resemble a ring. Lamarck has established this phylum. They are aquatic and occur on moist soil. A few of them are free living; some are burrowing and are parasitic too. The annelids show metameric segmentation in which their body is divided into external and internal segments. The external segments are divided by annuli and internal segments are formed by the transverse septa. These segments are called metameres and the first segment is called peristomium. They arise from the three germ layers such as ectoderm, mesoderm and endoderm. They are known as triploblastic animals. The coelomic fluid maintains the body shape and a true coelom is present. They are the first animals to have a true coelom. They have bilateral symmetry. They have an organ system level of organization. Their body wall is made up of cuticle, epidermis and muscular layer of circular and longitudinal muscles. The digestive system is complete and has mouth and anus for ingestion and egestion. The mode of respiration in most of the annelids is cutaneous and in few cases it is branchial. The vascular system is of closed type with the red color of blood which is attributed by haemoglobin. There are free amoeboid blood cells. However, in certain annelids like leech they do not have a true blood vascular system and there is a haemocoelomic system. They show direct development and has tube like structures known as nephridia which form the excretory system. Ammonia is their main excretory waste. They have unisexual and bisexual forms. They have the tactile and gustatoreceptors. These are receptors of touch and taste. They also have statocysts which act as a balancing organ. The nervous system is ring like with the presence of nerve cord and ganglia.
The annelids are considered better than the round worms due to the presence of true coelom, head appendages, the different types of muscle s, metameric segmentation and a red blood cell containing circulatory system.
