Found on beach in Atlantic Canada in November.
"Sack" is approx. 2" (approx. 5cm) in length.
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I've actually seen these before during an invertebrate zoology class in Maine! It's a rat-tailed sea cucumber, Caudina arenata. Though I don't have the taxonomic guide book anymore, I found this blog post that cites an old guide with more info about them.