LARVAL FORMS. 373 



5. Actinotrocha. The remarkable larva of Phoronis (fig. 

 230), known as Actinotrocha, is characterised by the presence of 

 (i) a postoral and somewhat longitudinal ciliated ring produced 

 into tentacles, and (2) a perianal ring. It is provided with a 

 prseoral lobe, and a terminal or somewhat dorsal anus. 



6. The larva of the Brachiopoda articulata (fig. 220). 

 The relationships of the six types of larval forms thus briefly 



characterised have been the subject of a considerable amount of 

 controversy, and the following suggestions on their affinities 

 must be viewed as somewhat speculative. The Pilidium type of 

 larva is in some important respects less highly differentiated 



FIG. 225. Two CH^TOPOD LARWE. (From Gegenhaur.) 



o. mouth ; i. intestine ; a. anus ; v. praeoral ciliated band ; w. perianal ciliated 

 band. 



than the larvae of the five other groups. It is, in the first place, 

 without an anus ; and there are no grounds for supposing that 

 the anus has become lost by retrogressive changes. If for the 

 moment it is granted that the Pilidium larva represents more 

 nearly than the larvae of the other groups the ancestral type of 

 larva, what characters are we led to assign to the ancestral form 

 which this larva repeats ? 



In the first place, this ancestral form, of which fig. 231 A is 

 an ideal representation, would appear to have had a dome-shaped 

 body, with a flattened oral surface and a rounded aboral surface. 

 Its symmetry was radial, and in the centre of the flattened oral 

 surface was placed the mouth, and round its edge was a ring of 

 cilia. The passage of a Pilidium-like larva into the vermiform 

 bilateral Platyelminth form, and therefore it may be presumed 

 of the ancestral form which this larva repeats, is effected by the 



