<P> The vertebral column consists of a centrum (the central body or spine of the vertebra), vertebral arches which protrude from the top and bottom of the centrum, and various processes which project from the centrum or arches . An arch extending from the top of the centrum is called a neural arch, while the hemal arch or chevron is found underneath the centrum in the caudal (tail) vertebrae of fish . The centrum of a fish is usually concave at each end (amphicoelous), which limits the motion of the fish . This can be contrasted with the centrum of a mammal, which is flat at each end (acoelous), shaped in a manner that can support and distribute compressive forces . </P> <P> The vertebrae of lobe - finned fishes consist of three discrete bony elements . The vertebral arch surrounds the spinal cord, and is of broadly similar form to that found in most other vertebrates . Just beneath the arch lies a small plate - like pleurocentrum, which protects the upper surface of the notochord, and below that, a larger arch - shaped intercentrum to protect the lower border . Both of these structures are embedded within a single cylindrical mass of cartilage . A similar arrangement was found in primitive tetrapods, but, in the evolutionary line that led to reptiles (and hence, also to mammals and birds), the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became the bony vertebral body . </P> <P> In most ray - finned fishes, including all teleosts, these two structures are fused with, and embedded within, a solid piece of bone superficially resembling the vertebral body of mammals . In living amphibians, there is simply a cylindrical piece of bone below the vertebral arch, with no trace of the separate elements present in the early tetrapods . </P> <P> In cartilagenous fish, such as sharks, the vertebrae consist of two cartilagenous tubes . The upper tube is formed from the vertebral arches, but also includes additional cartilagenous structures filling in the gaps between the vertebrae, and so enclosing the spinal cord in an essentially continuous sheath . The lower tube surrounds the notochord, and has a complex structure, often including multiple layers of calcification . </P>

Where is the heart of a fish located