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In fish, the system has only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single cycle circulation. The heart of fish is therefore only a single pump (consisting of two chambers). Fish have a closed-loop circulatory system.
From each segmental ganglion a branching system of local nerves runs into the body wall and then encircles the body. [8] However, in most polychaetes the two main nerve cords are fused, and in the tube-dwelling genus Owenia the single nerve chord has no ganglia and is located in the epidermis. [12] [36]
This is known as single cycle circulation. The heart of fish is, therefore, only a single pump (consisting of two chambers). [citation needed] In amphibians and most reptiles, a double circulatory system is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart. [citation needed]
Sharks possess a single-circuit circulatory system centered around a two-chambered heart. Blood flows from the heart to the gills where it is oxygenated. This oxygen-rich blood is then carried throughout the body and to the tissues before returning to the heart. As the heart beats, deoxygenated blood enters the sinus venosus.
Echinoderms are unusual in having both a coelomic circulatory system (the water vascular system) and a haemal circulatory system, as most groups of animals have just one of the two. [41] Haemal and perihaemal systems are derived from the original coelom, forming an open and reduced circulatory system. This usually consists of a central ring and ...
Exact details of the circulatory system are unclear, but the gut, pharynx, gills, gonads, and nervous system seem to be arranged in series rather than in parallel, as happens in most other animals. Every few minutes, the heart stops beating and then restarts, pumping fluid in the reverse direction. [12] Tunicate blood has some unusual features.
The group has the open circulatory system typical of arthropods, in which a tube-like heart pumps blood through the hemocoel, which is the major body cavity. Marine chelicerates have gills, while the air-breathing forms generally have both book lungs and tracheae.
The hagfish circulatory system also includes multiple accessory pumps throughout the body, which are considered auxiliary "hearts". [39] Hagfish are the only known vertebrates with osmoregulation isosmotic to their external environment. Their renal function remains poorly described. There is a hypothesis that they excrete ions in bile salts. [42]