Inside the heart of crayfish

The crustacean hearts are known to be neurogenic. The cardiac rhythm is originated in cardiac ganglion.
    The cardiac ganglion is composed of the pacemaker (small) and follower (large) neurons. The pacemaker neurons exhibit spontaneous bursting activity, and induce firing of follower neurons through excitatory synapses. The follower neurons make excitatory neuromuscular junctions onto the cardiac muscle and cause muscle contraction, the heartbeat.
    The cardiac muscles are simply driven by the motor neurons (followers) in the cardiac ganglion just like other skeltal muscles that are driven by their own motor neurons in the CNS.
    In crayfish, the cardiac ganglion is often buried under the cardiac muscle fibers.