Definitions:

1. A bony arch on the underside of a tail vertebra of a vertebrate, functioning as a protective covering over the ventral side of the spinal cord and blood vessels. NOUN
   • The development of haemal arches is a key feature in the evolution of the axial skeleton in aquatic vertebrates.
2. A bony or cartilaginous structure found in the vertebrae of some vertebrates, particularly fish and reptiles. NOUN
   • The haemal arch protects the caudal artery and vein as they pass through the tail of the fish.

Synonyms:

• blood vessel arch
• caudal arch
• hemal arch
• ventral arch
• vetebral arch