316 PHYLOGENETIC CONCLUSIONS. 



Echinoderm larvae differ from the Trochosphere, not only in the 

 points already alluded to, but in the character of the ciliated band. 

 The Echinoderm band is longitudinal and postoral. As just stated, 

 there is reason to think that the prseoral band of the Trochosphere and 

 the postoral band of the Echinoderm larva are both derived from a 

 ciliated ring surrounding the oral disc of the prototype of these 

 larvae {vide fig. 231). In the case of the Echinodermata the anus 

 must have been formed on the dorsal side of this ring, and in the 

 case of the Trochosphere on the ventral side ; and so the difference 

 in position between the two rings was brought about. Another view 

 with reference to these rings has been put forward by Gegenbaur 

 and Lankester, to the effect that the prseoral ring of the Trochosphere 

 is derived from the breaking up of the single band of most Echinoderm 

 larvae into the two bands found in Bipinnaria {vide fig. 223) and the 

 atrophy of the posterior band. There is no doubt a good deal to be 

 said for this origin of the praeoral ring, and it is 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 Coelenterata 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 ab- 

 sence 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 Trocho- 

 sphere larva, viz. the Eotifera, the Mollusca, the Chaetopoda, 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 pro- 

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



