Question

why is the body cavity of a cockroach not a true coelom but a haemocoel?

Answer 1

The body of some animals is made up of three layers of cells, the outer ectoderm, the middle mesoderm and the inner endoderm. The body cavity or the true coelom is the space enclosed by the mesoderm where the internal organs are suspended. Cockroaches also have true coelom.
However, the body cavity of cockroach is called as haemocoel as the body cavity remain filled with insect blood (haemolymph). They do not posses an arrangement of blood vessels as in mammals.