The body cavity in insects consists of sinuses containing blood in which all the organs lie bathed. We have already seen how air is carried to the tissues, rendering a respiratory pigment unnecessary. The blood is therefore colourless, but contains amceboid cells. It is caused to circulate by a tubular heart lying dorsal to the gut and enclosed by a very thin-walled pericardium. It consists of thirteen pear-shaped chambers, I.e. it is segmental in structure, having a hole at each side of the hinder end of each chamber. Each ostium is provided with a valve. There are many apertures in the pericardium allowing blood to get in from the body cavity, for there are no blood vessels. Such a system is an open one. When the heart ’beats, ’ a wave of contraction of its muscles begins at the hinder end and drives the blood in it forwards, the valves closing until the muscle relaxes. Then, aided by alary muscles, more blood enters from the pericardium. As blood is continually being forced into the sinuses at the anterior end, it gradually oozes backwards into the abdomen, eventually entering the pericardium once more.
The blood of the earthworm is carried entirely in ’vessels ’and capillaries, I.e. it is a closed one. The contractile dorsal blood vessel above the gut carries blood forwards ; from it five pairs of contractile pseudohearts convey blood downwards to ventral vessels that carry it backwards, giving off capillaries to all parts. Valves in the vessels prevent the blood from being forced the wrong way.
The blood is red, its haemoglobin being in solution. Numerous amoeboid cells are present.