The annelids or ringed worms are a large phylum of segmented worms with over 17,000 modern species including rag worms, earthworms, and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in fresh water and in moist, terrestrial environments. In most annelida species the circulatory system is closed. Characteristics of the circulatory system vary within the phylum. The blood usually contains hemoglobin, a red oxygen-carrying pigment; some annelids have a green oxygen-carrying pigment, and others have unpigmented blood. The circulatory system is usually closed, i.e., confined within well-developed blood vessels; in some polychaetes and leeches the circulatory system is partly open, with blood and coelomic fluid mixing directly in the sinuses of the body cavity. Blood flows toward the head through a contractile vessel above the gut and returns to the terminal region through vessels below the gut; it is distributed to each body compartment by lateral vessels. Some of the lateral vessels are contractile and serve as hearts, i.e., pumping organs for driving the blood.The hearts and the dorsal vessel have valves that prevent backflow.
The leech
The low pressure phases of contractions are responsible for the blood supply to the capillary networks(networks of small blood vessels and are parts of micro-circulation) of the organs and the body wall, with the possible exception of the intestine. The high pressure phases of contractions cause a forward bloodflow within the lateral vessel.Thedorsal and ventral vessels are longitudinal vessels. The dorsal vessel collects blood from the capillary networks of the organs and the body wall and discharges into the intestinal capillaries and the caudal laterodorsal vessels. The ventral vessel encloses the ventral nerve cord. Two lateral vessels also run along the entire length of the animal. They are contractile and drive the circulation, and are therefore called heart tubes.
The Earthworm
Earthworms have a simple closed circulatory system. They have two main blood vessels that extend through the length of their body: a ventral blood vessel which leads the blood to the posterior. end, and a dorsal blood vessel which leads to the anterior end. The dorsal vessel is contractile and pumps blood forward, where it is pumped into the ventral vessel by a series of "hearts" (aortic arches) which vary in number in the different taxa. The blood is distributed from the ventral vessel into capillaries on the body wall and other organs and into a vascular sinus in the gut wall, where gases and nutrients are exchanged. This arrangement may be complicated in the various groups by suboesophageal, Supraoesophageal, parietal and neural vessels, but the basic arrangement holds in all earthworms. Circulation in the earthworm is through a series of closed vessels. The two main vessels that can be seen in your dissection are the dorsal and ventral blood vessels. These vessels are the main pumping structures. In the dorsal vessel, blood moves anteriorly. The dorsal vessel is the dark line running along the dorsal surface of the digestive tract. In the posterior third of your worm, carefully cut through and remove about three centimeters of the digestive tract. The ventral blood vessel can usually be seen adhering to the segment of intestine removed. In the ventral vessel, blood moves posteriorly. Segmental branches off the ventral vessel supply the intestine and body wall with blood. These branches eventually break into capillary beds to pick up or release nutrients and, oxygen. Gas exchange occurs between the capillary beds of the body surface and the environment. Oxygen is carried by the respiratory pigment hemoglobin, which is dissolved in the fluid portion of the blood.
The Ragworm
The blood system is a closed system, meaning that they have blood vessels through which the blood flows. There are two main longitudinal blood vessels, a dorsal one and a ventral one. These vessels have circular muscles around them which can contract rhythmically to keep the blood moving around the body. Blood flows from the head to the tail in the ventral vessel and back, from the tail to the head, in the dorsal vessel. In each segment a number of smaller lateral (side) vessels branch off from the main vessels to supply the sections of that segment. The blood of rag worms contains hemoglobin, the same respiratory pigment as in humans, it is this that makes their blood the same red color as ours.