invertebrate with arms protruding from rock

[BayAreaSteve]

I have a few of these critters with black and white alternating striped "arms" that stick out of holes in the rocks. They seem to be increasing and do not move altho the arms remind me of brittle stars. There is one in the center of the pic and one on the upper right. They are about 1/2 inch with multiple arms protruding from the rock Anyone have an idea of what they might be? BTW have been enjoying the podcasts catching up with old ones and appreciate all the great stuff on TR. Thanks. Steve