Alary muscle, a pair (12) of triangular fan shaped muscle is present in the floor of pericardial sinus in each segment of cockroach. Blood circulates by the contraction and relaxation of heart assisted by alary muscles. Contraction of these muscles enlarges the pericardial sinus so that blood flows into it from the underlying perivisceral sinus. When these muscle relax, blood is forced through ostia into the heart.