A) The hepatic portal system is the venous system that returns blood from the digestive tract and spleen to the liver. It is present in mammals.

Both Hemichordata and Arthropoda has an open circulatory system. In this system the heart pumps blood into an open cavity called a hemocoel.

The circulatory patterns are of two types open or closed. Open circulatory system is present in arthropods and molluscs in which blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses. Annelids and chordates (example humans) have a closed circulatory system in which the blood pumped by the heart is always circulated through a closed network of blood vessels. This pattern is considered to be more advantageous, as the flow of fluid can be more precisely regulated.

Vertebrates, and a few invertebrates, have a closed circulatory system. Closed circulatory systems have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. In fish, blood flows from the heart to the gills for gas exchange, then to the rest of the body, and finally back to the heart. This is called a single circulation since the blood flows through the heart only once during each complete trip around the body. Amphibians evolved a double circulation; blood flows from the heart to the gills or lungs for gas exchange, then back to the heart to be repressurized before flowing to the rest of the body. The vessels that serve the respiratory organs are called the branchial circuit (for gills) or pulmonary circuit (for lungs). Vessels that serve the rest of the body are called the systemic circuit. The amphibian heart and most reptilian hearts have only three chambers, two atria and one ventricle and there is some mixing of oxygen-rich and oxygen-poor blood in the single ventricle. Endothermic vertebrates, the birds and mammals, have higher metabolic rates and require stricter separation of the pulmonary and systemic blood. Thus, they have four-chambered hearts. Oxygen-rich blood flows through the other ventricle to the systemic circuit.

An open circulatory system is present in arthropods in which blood is pumped by the heart passes through large vessels into open spaces or body cavities called sinuses. Annelids and chordates have a closed circulatory system in which the blood pumped by the heart is always circulated through a closed network of blood vessels.

In closed circulatory system, blood flows through closed chambers called as vessels. In open circulatory system, the blood flows in the closed vessel which further opens into a body cavity called haemocoel.

The circulatory system is the only system which is not exposed to the external environment.

The amphibians have blood directed in two circuits with two atria and one ventricle where mixing of blood takes place.

• Closed circulatory system has blood enclosed all the time within the vessels. In this system, blood is pumped by the heart through vessels.
• It is more structured and controlled.
  
  

Closed circulatory system has blood closed in all vessels within vessels of different sizes. In this type, the blood is pumped through vessels by heart. This type of circulatory system is more structured and controlled. 

An open circulatory system describes a system where blood and interstitial fluid (fluid found between cells in the body) are allowed to mix in an organism. Organisms that have an open circulatory system don't have true blood since it is mixed with other fluids. The blood in these organisms is defined as haemolymph. Two of the largest phyla in the world, arthropoda and mollusca, have an open circulatory system. Arthropods include most insects and sea organisms like crabs and lobsters. Molluscs are organisms with two shells that are hinged together. Examples include clams and oysters. However, not all molluscs have an open circulatory system. Two major members of the mollusc family, octopus and squid, have a closed circulatory system. 

Insects have an open circulatory system, which differs in both structure and function from the closed circulatory system found in humans and other vertebrates. In an open system, blood (usually called haemolymph) flows freely within body cavities, where it makes direct contact with all internal tissues and organs.

Leeches belong to Phylum Annelida and have closed circulatory systems that carry blood, nutrients etc to all parts of the body.

In a closed circulatory system, blood is always contained within vessels - arteries, veins, capillaries.

In an open circulatory system, the blood is not confined within blood vessels. The blood bathes the organs and tissues directly. Because of this the blood and the interstitial fluid have no distinction. Thus, the term hemolymph is used. The hemolymph is pumped by the heart through the vessels into the sinuses and then to the surrounding cells where the exchange of materials occur. The hemolymph is taken back to the heart through the ostia that close via the valves during heart contraction.

Oxygen carrying pigment haemoglobin are present as separate entities and not found dissolved in plasma like inorganic salts. 

The open circulatory system is common to molluscs and arthropods. Open circulatory systems (evolved in crustaceans, insects, mollusks and other invertebrates) pump blood into a hemocoel with the blood diffusing back to the circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded by the blood. So, the blood is not segregated from interstitial fluid.