Complex sentence: 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.
Split into simpler sentences: 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. This makes them less rigid and more able to move to follow the tides.