Figure 32.11: The Heart of a Lobster (A) The lobster heart is stimulated to beat by nerve impulses that originate in a cardiac ganglion (yellow) associated with the dorsal heart wall. (B) Blood enters the single heart chamber via slits in the heart wall called ostia. These slits act as one-way valves, allowing blood to enter the heart chamber but not exit. When the heart contracts, blood inside is forced into the arteries. Contraction also stretches the elastic ligaments. After contraction, when the heart muscle relaxes, the stretched ligaments pull the heart wall outward as they shorten, thereby sucking blood into the heart chamber through the ostia.