Here's the entire animal:
I was finishing up some field work in the rocky intertidal zone on Bodega Head late this afternoon, and was a little surprised to encounter this small octopus on top of the mussel bed.
On this warm day, it looked like the octopus would rather be elsewhere (although that's an assumption on my part), so I picked it up and placed it in a nearby tidepool.
Upon entering the pool, it was positioned looking up towards me (see below). From this angle, you could easily see the siphon — it's located between the right eye and the lowest arms. The siphon aids in jet propulsion; when water is forced out of the mantle cavity through this funnel-like structure, the animal moves in the opposite direction. If an octopus is threatened, a dark cloud of ink may also be released through the siphon. The ink acts as a "smoke screen," allowing the octopus to escape, and may also irritate predators such as fish.
Fairly quickly, the octopus moved to the bottom of the pool, providing some very nice views from above.
The next photo illustrates how octopus can change not only the color but also the texture of their skin. In this case, it now appears rough and bumpy.
This is the last view I had before a wave washed over the pool and I could no longer see the octopus. Look for the suckers on the bottoms of the arms. It's well equipped for catching and manipulating prey such as small crabs and hermit crabs.
I don't have much experience identifying octopus. This might be a Red Octopus (Octopus rubescens). However, let me know if you have other thoughts about the identification!