The animal in this picture is a fairly advanced larva of an unknown species of phoronid worm.  This distinctive larval form is called an actinotroch.  The phoronids are among the animal phyla least likely to have been encountered by the ordinary citizen: most of them are rather diminutive, infaunal worms that have never been known to trouble anyone of importance — except, of course, for zoologists interested in the evolution of animal form, for whom the phoronids remain an enduring puzzle.

From bottom to top: the actinotroch motors itself through the water using a dense band of large cilia – called the telotroch – that encircles the very posterior end, where the larval digestive tract ends.  The body column is muscular and flexible, so the actinotroch can orient its swimming the same way one would steer a boat with an outboard motor. 

Most of that flexible body column is filled with an immense stomach, but what is most prominent in this picture is a string of orange jewels decorating the edge of an internal sac which will eventually give rise to much of the adult body.

To fill its enormous stomach, the actinotroch uses its distinctive array of flexible, ciliated tentacles.  Most of the cilia on the tentacles beat to create a water current, but a few are stiff, and these create a sieve.  Thus the tentacles trap small algal cells (and other objects).  The tentacles flick upward to deliver trapped particles to the mouth, which is underneath a large muscular hood – the dome on the top.  Richard Strathmann, who described their feeding mechanism from high-speed video, noted that the actinotroch can also gulp in prey by rapidly flapping the hood upward.

This particular larva is practically a macroscopic animal: it’s almost 3 mm long.  Like many larvae, it is much more than simply the embryo of an adult.  It has behavior, interests of its own, and probably a relatively long life in the plankton in which to explore those interests.  I can’t say what species of phoronid it belongs to because, as remains the case for many planktonic larvae, this one has not been matched to the adult it will some day become.  Only the larva knows for sure.