The image likely shows the base of a brown algae, such as Saccorhiza polyschides (also known as featherweed or furbellow) or Laminaria.
This is a type of seaweed that often washes up on beaches.
The knobby structure is the sturdy 'root' (attachment plate) with which the algae attaches itself to the seabed.
It is not an animal, like a sea urchin, despite its spiny appearance.