Convergence

  

The underside of a Pacific Sand Dollar (Dendraster excentricus), showing the amazing food grooves leading to the mouth (close to the center of the sand dollar).

The grooves look like tree branches or tributaries:

 

 

You can also see the tiny white knobs that the sand dollar's spines would be attached to when alive:

 

 

One more, showing a close-up of the convergence of the food grooves at the mouth:

 

 

 Photographed in Crescent City, CA, on 27 October 2022.

 

P.S.  For a little more info about food grooves, see the post called "Petals and grooves" from 11 September 2012.