This sea cucumber (Cucumaria pseudocurata) is found in the rocky intertidal zone along our coast. Recently we encountered quite a few of them nestled in shallow depressions below mats of algae.
In the field, the sea cucumbers look like little brown blobs (maximum size is ~3.5 cm, or ~1.5 inches):
As you probably saw in the video, when viewed under a microscope their features are easier to see, including tubefeet (for holding on), tentacles (for gathering food), and large shiny ossicles (for skeletal support):
Ossicles are plates made of calcium carbonate. The shape of the ossicles helps distinguish different species of sea cucumbers. Below is a selection of ossicles from Cucumaria pseudocurata viewed under very high magnification:
You can compare the real thing (above) with a book illustration (below) and look for similarities:
Modified from Sea Cucumbers of British Columbia, Southeast Alaska, and Puget Sound (Lambert 1997)
And here's a wonderful close-up view of the ossicles inside the body wall and a tubefoot:
Perhaps you remember that the sea cucumber in last night's video was only ~3 mm long. Cucumaria pseudocurata is a brooder and a direct developer — females lays eggs in January, males release sperm to fertilize the eggs, the female shields the developing embryos with her body for about a month, and eventually tiny juvenile sea cucumbers are visible among the adults.
The individual in the video was probably about 3-4 months old. Here's a still picture with a ruler to help you visualize the size (the marks on the ruler represent millimeters):
And in case you missed it, here's the video: