Not closed blood system is characterized by the fact that blood streams from vessels directly in a body cavity. After that it gathers in vessels again. Such blood system from all animals only mollusks and arthropods possess.
Blood system of mollusks
Not closed blood system is available for mollusks. These are water or land animals which body mainly consists of soft fabrics and it is covered with a sink. The body cavity at adult individuals is substantially reduced, and intervals between bodies are filled with connecting fabric. The blood system includes heart and vessels, heart is divided into 1 ventricle and several auricles. Auricles can be 2 or 4, and there can be also only one.
From vessels blood streams in intervals between internals where gives oxygen then gathers back in vessels and goes to respiratory organs. Respiratory organs – a lung or gills, are covered with dense network of capillaries. Here blood is saturated with oxygen again. Blood of mollusks generally has no color, it contains the special substance capable to contact oxygen.
Exception are the cephalopod mollusks having almost closed blood system. Hearts at them two, are located both hearts in gills. Blood on capillaries of gills moves, then from the main heart arrives to bodies. Thus, blood flows in a body cavity partially.
Blood system of arthropods
Not closed blood system as well at type Arthropods which representatives inhabit all possible habitats. Characteristic of arthropods – existence of the chlenisty extremities allowing to make various movements. Classes belong to this type: Cancroid, Arachnoid, Insects. There is a heart located over intestines. It can have an appearance both a tube, and a sack. From arteries blood gets into a body cavity where gives oxygen. Gas exchange becomes possible, thanks to existence in blood of a respiratory pigment. After blood gathers in veins and comes to branchiate capillaries where it is saturated with oxygen. At Crustacea the structure of a blood system is directly connected with the building respiratory. Heart at them is near respiratory organs. At primitive Crustacea the heart has a tube appearance with openings in each segment of a body, at more developed - a type of a sack. There are primitive Crustacea at whom gas exchange happens through a body wall. At such the blood system can be absent in general. Heart arachnoid generally has a tube appearance with several couples of openings. At the smallest it has a sack appearance. The liquid moving on the blood system of insects is called a haemo lymph. It partially is in special body – the back vessel having a tube appearance. The rest washes internals. The back vessel consists of heart and an aorta. Heart is divided into cameras, their quantity corresponds to number of segments of a body.