What is haemocoel? Which group of animals has haemocoel?

A hemocoel (or haemocoel) is a series of spaces between the organs of organisms with open circulatory systems, like most arthropods. It is a body cavity, containing blood, which has the same embryonic origins as the blood-vascular system (i.e. it is not a coelom). Functionally, it replaces the coelom as the main body cavity and hydrostatic skeleton but it does not contain gametes.

The haemocoel is the main body cavity of invertebrates, such as insects which belong to kingdom arthopoda.

Insects do not possess an arrangement of blood vessels such as those in mammals. Instead insects have a cavity that is full of insect blood (known as haemolymph) and the organs of the insect are suspended in this cavity. This means the organs do not have a specific blood supply but are suspended in the blood and receive nutrients as the blood 'washes' around the organs.

The insects can be housefly, butterfly, scorpion, spider etc.