Pacific Sand Dollars (Dendraster excentricus) can be found washed ashore on both Doran Beach and Salmon Creek Beach. I happened upon this relatively small individual (~2 cm across) on Salmon Creek Beach.
Sand dollars are related to sea urchins. Although they are flattened, they show the same five-part symmetry common to echinoderms ("spiny-skinned" animals, a group that also includes sea stars and sea cucumbers).
Remember that what you find on the beach is an endoskeleton (internal skeleton). When alive, the animal lives partially buried in the sand offshore and is covered with very short, dense, movable spines.
There are lots of interesting things to see on a sand dollar skeleton (also known as a test). The top side (shown above) is the aboral surface (aboral means opposite the mouth). The large petal-like structures are called petaloids.
If you use a magnifier to look at them closely, you'll see that the edges of the petaloids are actually paired holes or pores connected by a narrow groove (see next photo). These pores allow respiratory tube feet to emerge from the test. The tube feet lay flat within the grooves and are specialized to assist with the exchange of oxygen. The petaloids help the sand dollar breathe!
In the center of the petaloids is a small perforated plate called the madreporite. There are four large pores surrounding the madreporite. These are gonopores that release eggs or sperm.
Also in the photo above, look closely for the small white tubercles (bumps) scattered across the entire surface of the sand dollar. These are the attachment points for the spines. (I'll try to find a live sand dollar and will post a photo if possible!)
Here's a picture of the bottom side, referred to as the oral surface because it supports the mouth.
The large opening in the center is called the peristome. The mouth is at the center of the peristome in a living sand dollar. The smaller opening near the back edge is called the periproct (it's slightly broken in this specimen); it surrounds the anal opening.
Note that there are five grooves radiating away from the mouth. These are food grooves. They branch quickly and are even more noticeable on older specimens (see below).
The sand dollar transports food particles along these grooves towards the mouth. The spines and tube feet capture food particles, and then the tube feet move the particles along the grooves like a conveyor belt (see diagram below).
Modified from Invertebrate Zoology, Fifth Edition (Barnes 1987)