Annelids the triploblastic protostomes have a body cavity called the coelom. Triploblastic organisms meaning the species will have three primary tissue areas that are formed during the embryogenesis stage. It is a fluid filled cavity within which the gut and the other organs are suspended. The body cavity is separated into a serious of compartments separated by the wall called as septa. In most of the species each compartment is considered as a single segment which encompasses a part of the nervous and circulatory system making them to function independently. Annuli a ring like structure marks each segment. Each segment has a circular muscle as an outer layer below the thin cuticle and the epidermis layer and also consists of a londitudinal muscle system.
Annelids have a closed circulator system and posses true segmentation. They have a bilateral symmetrical body.
The longitudinal muscles are strengthened by collagenous lamellae in the earthworms and the leeches have a double layer of muscles between the outer circular muscles and inner longitudinal muscle. Most of the species in the annelids carry a bristles called the setae and parapodia a pair of appendages.
In front of the true segment is the prostomium and peristomium that carry the mouth and at the back of them lies the posterior lies the pygidium the place where the anus is located. The digestive system is quite uneven but it is specialized.
The annelids have a vary diet depending on the species which also includes the active and passive hunters, direct feeders, filter feeders and scavengers. The digestive system is separate from the vascular system and the nervous system. The vascular system contains a dorsal vessel that conveys the blood towards the front part of the worm. It also includes a ventral longitudinal vessel that conveys the blood in the opposite direction. The dorsal vessel and the ventral longitudinal vessel are linked by the vascular sinus and by different kinds of lateral vessels.
The lateral nerves rises in each segment from the solid ventral nerve cord that is located in the nervous system. Every segment in the species unites to perform a single function like locomotion. Growth in many species is done by duplicating some individual segments and in other species the number of segments is fixed early in the development stage itself.