You're right, a sea urchin. Looks like a good match for Echinometra mathaei images in guide books. They can wedge themselves into really tight places and also bore holes into reef rock m in which they live.
It's a Lithopoma, a starsnail as Deborah said. There are a couple of different species which are very hard to tell apart without having the animal in hand.
Hi Lowell -- It's also a sea cucumber. Either the feeding tentacles are retracted into the body or they're otherwise out of sight. If you do an image search for Euapta, Opheodosoma, or Synaptula you'll find pictures of similar animals.
Striped gooseneck barnacle, Conchoderma virgatum. It's found world wide in the tropics, attached to things like buoys, ships, offshore structures as well as the big oceanic species like whales, sunfish, and turtles.
mtis -- I think your french forum friend was right. Here's a page on Actinia fragacea with images that look very similar to yours.
http://www.asturnatura.com/especie/actinia-fragacea.html
what a pity, worms have so much more class than anemones! :)
It certainly looks like a flatworm that has eaten something - that's why the midsection is bulging. Many of them are mollusc-eaters and have one of two types of feeding modes. The first is to slip into the shell of the mollusc and slurp up the inhabitant. Oyster & clam specialists use that...
Err, Pseudoceros sapphrinus occurs in the western Indo-Pacific. A sighting off southern California is kinda a stretch. :) Besides, that has a bright blue band around the margin.
It depends what country you're from. At conferences it's been very obvious that americans pronounce scientific names differently from other english-speakers. The species name is constructed from "longi" and "cirrum" so I (american that I am) would use lon-ji-YR-um.
The first 3 are of the cast off molt of a barnacle. Like all crustaceans they periodically shed their old carapaces in order to grow larger. the second is a small jelly, possibly in the genus Euphysa or Sarsia. See the Jellies Zone -
Sarsia and Euphysa
Hi Sue -- Thanks for your interest. To prepare a fresh-caught sea star it needs to be first narcotized so it doesn't curl into a ball then put into 80% alcohol. If it's large it will need alcohol injected into the body to make sure the internal organs are preserved as well as the outside...
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