There are at least three types of muscles in crustacean hearts. The cardiac
muscle, the ostium muscle and the muscles of cardio-arterial
valves. Ostium (pl.ostia) is a valvular opening on heart wall,
from which blood (hemolymph) flows into the heart...
Chronic recording of cardio- regulation in free-living crayfish
There are only a pair of cardio-inhibitory nerves and a pair of
nerves projecting out from the CNS to the heart. The heartbeat is always
under control by these nerves and hormones.