On quitting the shelter of the parent tentacles the embryo becomes a pelagic larva, known as Actinotrocha (fig.
Phoronis has long been regarded as a possible ally of Rhabdopleura (see Pterobranchia); and Masterman (to) has attempted to demonstrate the existence in Actinotrocha of most of the structures which occur in the Pterobranchia.
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.
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.
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.
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.
According to his view the praeoral hood of Actinotrocha (cf.
It has, moreover, been shown (see especially Goodrich, 5) that shortly before its metamorphosis, Actinotrocha develops a coelomic space which lies immediately in front of the oblique septum, and gives rise later to the cavity of the lophophore and tentacles.
After the Actinotrocha has led a pelagic, life for some time it develops a large ventral invagination of its body-wall (fig.