This is just a quick & simple mirror & merge of tc_fea's half nautilus shell for Project Shellter.

For info regarding Project Shellter, see:
http://www.makerbot.com/blog/2011/10/18/project-shellter-can-the-makerbot-community-save-hermit-crabs/

Some internal support needed.

I do not have access to a hermit crab to test functionality, but the photo in the blog linked above shows a hermit crab using a broken bottle as a shell, so they're not picky.