The thecates have a solid structure (theca) like a skeleton around the tentacles, into which the tentacles can retreat, the athecates have no such structure which makes them less rigid and more able to move to follow the tides.
The thecates have a solid structure (theca) like a skeleton around the tentacles, the athecates have no such structure. This such structure makes them less rigid and more able to move to follow the tides. The tentacles can retreat into the tentacles.