Such a form as this is roughly represented to-day by the Actinotrocha larva of Phoronis, the importance of which has been brought out by Masterman.
PHORONIDEA, a zoological order, containing a single genus Phoronis, which is known to be of practically world-wide distribution, while there are many records of its larva, Actinotrocha, from localities, where the adult has not been found.
The peduncle of the Brachiopoda was supposed to correspond with the everted ventral sac of Actinotrocha, but the question is complicated by the want of any complete investigation of the development of the Brachiopoda, and by the absence of the anus in the majority of the genera.
Most observers consider that Actinotrocha is a highly modified Trochosphere, and this would give it some claim to be regarded as distantly related to the Entoproct Polyzoa and to other groups which have a Trochosphere larva.
One of the most formidable difficulties in the way of the attempt to reduce Actinotrocha to the Pterobranchiate type of structure is the condition of the coelom in the former.
The epistome of the adult Phoronis cannot well be the proboscis since its cavity is continuous with the lophophoral coelom, and because the praeoral hood of Actinotrocha is entirely lost at the metamorphosis.