384 PHYLOGENETIC CONCLUSIONS. 



strengthened by the case of Tornaria ; but the view adopted 

 above appears to me more probable. 



Actinotrocha (fig. 230) undoubtedly resembles more closely 

 Echinoderm larvae than the Trochosphere. Its ciliated ring has 

 Echinoderm characters, and the growth along the line of the 

 ciliated ring of a series of arms is very similar to what takes 

 place in many Echinoderms. It also agrees with the Echinoderm 

 larvae in the absence of sense organs on the praeoral lobe. 



Tornaria (fig. 229) cannot be definitely united either with 

 the Trochosphere or with the Echinoderm larval type. It has 

 important characters in common with both of these groups, and 

 the mixture of these characters renders it a very striking and 

 well-defined larval form. 



Phylogenetic conclusions. The phylogenetic conclusions 

 which follow from the above views remain to be dealt with. 

 The fact that all the larvae of the groups above the Ccelenterata 

 can be reduced to a common type seems to indicate that all the 

 higher groups are descended from a single stem. 



Considering that the larvae of comparatively few groups have 

 persisted, no conclusions as to affinities can be drawn from the 

 absence of a larva in any group; and the presence in two groups 

 of a common larval form may be taken as proving a common 

 descent, but does not necessarily shew any close affinity. 



There is every reason to believe that the types with a 

 Trochosphere larva, viz. the Rotifera, the Mollusca, the Chaeto- 

 poda, the Gephyrea, and the Polyzoa, are descended from a 

 common ancestral form ; and it is also fairly certain there was a 

 remote ancestor common to these forms and to the Platyelminthes. 

 A general affinity of the Brachiopoda with the Chaetopoda is 

 more than probable. All these types, together with various 

 other types which are nearly related to them, but have not 

 preserved an early larval form, are descended from a bilateral 

 ancestor. The Echinodermata, on the other hand, are probably 

 directly descended from a radial ancestor, and have more or less 

 completely retained their radial symmetry. How far Actino- 

 trocha 1 is related to the Echinoderm larvae cannot be settled. 

 Its characters may possibly be secondary, like those of the 



1 It is quite possible that Phoronis is in no way related to the other Gephyrea. 



