ACK! Our systems are still having issues and it's taking much longer than expected to fix it... but rest assured we ARE actively working
on restoring everything to 100%. We really appreciate everyone's patience... and we also have a major surprise coming soon! *hint, hint*
The structure of the heart can vary among the different branches of the animal kingdom. (See Circulatory system.) Cephalopods have two "gill hearts" and one "systemic heart". In vertebrates, the heart lies in the anterior part of the body cavity, dorsal to the gut. It is always surrounded by a pericardium, which is usually a distinct structure, but may be continuous with the peritoneum in jawless and cartilaginous fish. Hagfishes, uniquely among vertebrates, also possess a second heart-like structure in the tail.