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At the centre of a worm is the digestive tract, which runs straight through from mouth to anus without coiling, and is flanked above and below by blood vessels (the dorsal blood vessel and the ventral blood vessel as well as a subneural blood vessel) and the ventral nerve cord, and is surrounded in each segment by a pair of pallial blood ...
Blood is carried forward in the dorsal vessel (in the upper part of the body) and back through the ventral vessel (underneath), before passing into a sinus surrounding the intestine. Some of the smaller vessels are muscular, effectively forming hearts; from one to five pairs of such hearts is typical.
Acorn worms have an open circulatory system, in which the blood flows through the tissues sinuses. A dorsal blood vessel in the mesentery above the gut delivers blood to a sinus in the proboscis that contains a muscular sac acting as a heart. Unlike the hearts of most other animals, however, this structure is a closed fluid-filled vesicle whose ...
Earthworm head. Lumbricus terrestris is relatively large, pinkish to reddish-brown in colour, generally 110–200 millimetres (4.3–7.9 in) in length and about 7–10 millimetres (0.28–0.39 in) in diameter. It has around 120–170 segments, often 135–150. The body is cylindrical in the cross section, except for the broad, flattened ...
Hemichordata (/ ˌhɛmikɔːrˈdeɪtə / HEH-mee-kor-DAY-tə) is a phylum which consists of triploblastic, eucoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta (acorn worms), and ...
The two main blood vessels furnish smaller vessels to supply the parapodia and the gut. Blood flows forward in the dorsal vessel, above the gut, and returns down the body in the ventral vessel, beneath the gut. The blood vessels themselves are contractile, helping to push the blood along, so most species have no need of a heart.
Microchaetus rappi, the African giant earthworm, is a large earthworm in the family Microchaetidae, the largest of the segmented worms (commonly called earthworms). It averages about 1.4 meters (4.5 feet) in length, but can reach a length of as much as 6.7 meters (22 feet) and can weigh over 1.5 kilograms (3.3 pounds).
The circulatory systems of all vertebrates, as well as of annelids (for example, earthworms) and cephalopods (squids, octopuses and relatives) always keep their circulating blood enclosed within heart chambers or blood vessels and are classified as closed, just as in humans.