Again a larva. I dont know from which animal it comes. Maybe from a polychaeta. Somebody can help me? (The marine larvae are so complicated, but nevertheless I like them).
I think again that this is the trochophore larva of a polychaete. This is an early stage with the trochophore developing within the egg "shell". This type of development, with the cilia going out through the shell and the very thick shell is typical of the Spionidae.
Your photo shows the cilia so well - I am sure not an easy image to take.
just a thought.... how about trying to rear some of your larvae. It can be quite simple to rear early stage larvae. It would be fascinating to see how they develop and it might make identifying them easier. Equipment might be as simple as a few more champagne glasses !
There is a book you might like to get:
"Reproduction and Development of Marine Invertebrates of the Northern Pacific Coast" MF Strathmann (2000).
It tells you all you need to know about spawning and rearing marine invertebrate larvae - it is full of practical info . It will also help with IDs.
The first edition was published in the 80´s ...I used to have a copy...but then I lent it to someone!