![]() The nervous system has a nerve cord from which lateral nerves come in contact with each segment. The two systems are connected by a vascular sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body wall. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel which conveys the blood in the opposite direction. The vascular system and the nervous system are separate from the digestive tract. Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders which simply ingest the sediments, and blood-suckers. For example, in some groups (notably most earthworms) it has a typhlosole (to increase surface area) along much of its length. The digestive tract is quite variable but is usually specialized. Schleiden (1804–1881)Īnterior to the true segments lies the prostomium and peristomium, which carries the mouth, and posterior to them lies the pygidium, where the anus is located. In most forms they also carry a varying number of bristles, called setae, and among the polychaetes a pair of appendages, called parapodia.įile:Libr0409.jpg Polychaeta: "A variety of marine worms" plate from Das Meer by M. In earthworms and in daria the longitudinal muscles are strengthened by collagenous lamellae the leeches have a double layer of muscles between the outer circulars and inner longitudinals. Each segment also has an outer layer of circular muscle underneath a thin cuticle and epidermis, and a system of longitudinal muscles. Each segment (metamere) is marked externally by one or more rings, called annuli. Tiny blood vessels are abundant in the earthworm's skin, which function as its respiratory organ. The dorsal vessel and five pairs of vessels that circle the esophagus of an earthworm are muscular and pump blood through the circulatory system. Dorsal and ventral vessels are connected by segmental pairs of vessels. The closed circulatory system consists of networks of vessels containing blood with oxygen-carrying hemoglobin. In the most general forms each compartment corresponds to a triple segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. ![]() The coelom is divided into a sequence of compartments by walls called septa. Oligochaetes and polychaetes typically have spacious coeloms in leeches, the coelom is filled in with tissue and reduced to a system of narrow canals archiannelids may lack the coelom entirely. Their segmented bodies and coelom have given them evolutionary advantages over other worms. Annelids are bilaterally symmetric and triploblastic protostomes with a coelom (which makes them coelomates), closed circulatory system and true segmentation.
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