384 



INVEETEBRATA 



CHAP. 



If Caldwell has given the details of the metamorphosis correctly 

 it is exceedingly ditiicult to interpret, for his account would seem 

 to imply that the apical plate and subjacent ganglion are sacrificed, 

 in which case the cerebral ganglion of the adult must be a new 



B 



irur 



aten 



FIG. 311. Three stages in the meta- 

 morphosis of the Actinotrocha 

 larva of Phoronis, seen from the 

 side. (After Metschnikoff. ) 



A, stage in which the ventral ecto- 

 dennic imagination has just made its 

 appearance. B, stage in which the ventral 

 invagination is' partly everted. C, stage 

 in which the metamorphosis is almost 

 complete, a.ten, rudiments of adult 

 tentacles ; gl, glandular pocket of the 

 stomach regarded by Masterman as a 

 homologue of the notochord ; int, intes- 

 tine ; inv, ectodermic invagination which 

 becomes everted to form the body of 

 the worm ; l.ten, larval tentacles of the 

 metatroch ; pr .1, prae-oral lobe ; st, 

 stomach ; t.tr, telotroch. 



formation. Now in the metamorphosis of every Trochophore so far 

 studied, the apical plate and the associated ganglion form the head- 

 blastema, and persist through larval life into the adult condition. 

 It is possible that Caldwell has made a mistake in this matter, and 

 that it is the hood in front of the ganglion, which we have already 

 compared to a broad prototroch bearing minute cilia, which is 



