Final answer:
A common excurrent siphon is called a siphuncle, which is a tube-like structure found in certain marine organisms for buoyancy control.
Step-by-step explanation:
A common excurrent siphon is called a siphuncle. In biology, a siphuncle refers to a tube-like structure found in certain marine organisms, such as chambered nautiluses and some cephalopods. The siphuncle is responsible for regulating the gas-filled chambers within the organism's shell, allowing for buoyancy control.