An open circulatory system is characterized by the fact that blood is poured from the vessels directly into the body cavity. After that, it is reassembled into vessels. Of all animals, only mollusks and arthropods have such a circulatory system.
The circulatory system of molluscs
An open circulatory system is found in molluscs. These are aquatic or terrestrial animals, the body of which consists mainly of soft tissues and is covered with a shell. The body cavity in adults is largely reduced, and the gaps between the organs are filled with connective tissue. The circulatory system includes the heart and blood vessels, the heart is divided into 1 ventricle and several atria. There may be 2 or 4 atria, or there may be only one.
From the vessels, the blood is poured into the gaps between the internal organs, where it gives off oxygen, after which it is collected back into the vessels and sent to the respiratory organs. Respiratory organs - lung or gills, covered with a dense network of capillaries. Here the blood is again saturated with oxygen. The blood of molluscs is mostly colorless; it contains a special substance that can bind to oxygen.
The exception is cephalopods, which have an almost closed circulatory system. They have two hearts, both hearts are located in the gills. Blood moves through the capillaries of the gills, then from the main heart it flows to the organs. Thus, the blood flows out into the body cavity partially.
Arthropod circulatory system
An open circulatory system also occurs in the Arthropod type, whose representatives inhabit all possible habitats. A characteristic feature of arthropods is the presence of articulated limbs, which allow them to perform various movements. This type includes the following classes: Crustaceans, Arachnids, Insects.
There is a heart located above the intestines. It can be in the form of both a tube and a bag. From the arteries, the blood enters the body cavity, where it gives off oxygen. Gas exchange becomes possible due to the presence of respiratory pigment in the blood. After that, the blood is collected in the veins and enters the gill capillaries, where it is saturated with oxygen.
In crustaceans, the structure of the circulatory system is directly related to the structure of the respiratory system. Their heart is located near the respiratory system. In primitive crustaceans, the heart looks like a tube with holes in each segment of the body; in more developed crustaceans, it looks like a sac. There are primitive crustaceans in which gas exchange occurs through the body wall. In these, the circulatory system may be absent altogether. The heart of arachnids is basically a tube with several pairs of holes. In the smallest, it looks like a bag.
The fluid that moves through the circulatory system of insects is called hemolymph. It is partially located in a special organ - the dorsal vessel, which looks like a tube. The rest washes the internal organs. The dorsal vessel consists of the heart and the aorta. The heart is divided into chambers, their number corresponds to the number of body segments.
