394 F. M. BALFbUR. 
on the praeoral lobe, in the existence of a perianal ring of 
cilia, and in the possession of a contractile band passing 
from the praeoral lobe to the oesophagus. 
5. Actinotrocha. — The remarkable larva of Phoronis, 
(fig. 17) known as Actinotrocha, is characterised by the 
presence of (1) a postoral and somewhat longitudinal ciliated 
ring produced into tentacles, and (2) a perianal ring. It 
is provided with a praeoral lobe, and a terminal or some- 
what dorsal anus. 
6. The larva of the Brachiopoda articulata (fig. 6). 
The relationships of the six types of larval forms just 
characterised have been the subject of a considerable 
amount of controversy, and the following suggestions on 
the subject must be viewed as somewhat speculative. The 
Pilidium type of larva is in some important respects less 
highly differentiated than the larvse 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 ? 
Fig. 12. — Two Chcetopod Larva.^ (From Gegenbaur.) o. mouth ; i. intes- 
tine ; a. anus ; v. prseoral ciliated band ; w. perianal ciliated band. 
In the first place, this ancestral form, of which fig. 18 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 larva becoming more 
elongated, and by the region between the mouth and one 
