Animal, Vegetable, or Mineral?

Here are pictures of a creature (egg case? plant? invertebrate?) my family has often seen in shallow water near Block Island, RI, during the first week of August. After failing to describe it adequately, we swam out and dug one up for photography. And science!

Here it is sitting in a 1-quart plastic tub:



Here's a somewhat closer view:



Found attached to sandy bottom in approx. 4-6 ft. of water (high tide), Block Island, RI. Approx. 3-4 inches long. In some years they are spaced quite regularly across the bottom, not closer together than about a foot. Only the narrow end is attached to the bottom, so the whole thing swings with the motion of the waves. The attachment or root structure doesn't seem like much in particular, just stringy jelly-like stuff.

It's possible to look in the narrow end, through an opening perhaps 1cm in diameter. The interior of the whatsit looks like an empty bag. The clear skin (?) or shell or creature is soft, resilient, and bouncy. It looks to me about a quarter centimeter thick. The brownish color is actually little brown specks floating around in this jelly. We also found a blob where the brown specks had apparently collected into a few much larger spots, though we couldn't decide if this was due to growth natural to the whatsis, or to infection or damage.

These things are not watery inside, but jelly-like -- the children found this out by squeezing one too hard.

The specific location is the extreme southern end of Crescent Beach, just north and west of the Old Harbor jetty, just below the Surf Hotel.

It's been suggested that these might be squid egg cases of some sort, but Google has coughed up nothing that looks like this.