First, here's a nice image showing a newly hatched actinula larva next to an empty egg case (the egg case is clear and partially torn). I'm guessing the larva had emerged from this case very recently. (If you look closely, you can see that the egg case is being held from below by a slender clasper tentacle.)
Next, two of my best pictures of a young Candelabrum fritchmanii larva. These images are of the same individual, but they show how extensible the larva is — stretching outward and pulling back in. This larva is ~1 mm long.
When the larvae are about a week old, the posterior region elongates and develops an attachment disc at the end. Here are two different individuals, the first with the attachment disc at the bottom of the photo (a bit blurry), and the second with the disc at the top.
The next stage in the transformation is dramatic. Once the larva attaches to the substrate (in this case the substrate is kelp), it basically collapses into a sphere. During this process of metamorphosis, the long primary larval tentacles are resorbed, so all you see are short stubby tentacles:
And then it very quickly becomes a tiny polyp! Two days after I took the picture above, here is what this polyp looked like:
It was amazing to see the process from hatching to a crawling actinula larva with long outstretched tentacles to an attached rounded sphere and then a tiny polyp, all within about a week and a half!