LARVAL FORMS ; THEIR NATURE, ORIGIN AND AFFINITIES. 405 
Actinotrocha (fig. 17) undoubtedly agrees more closely with 
Echinoderm larvse than with the Trochosphere. Its ciliated 
ring has the same character, 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. Its affinity with 
the Echinoderm larvae is also shown in the absence of 
sense organs on the praeoral lobe. 
Tornaria (fig. 16) 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 Coelen- 
terata can be reduced to a common type points to all the 
higher groups being 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 ; and the presence in two groups 
of a common larval form may be taken as proving a 
common descent, but does not necessarily show any close 
affinity. 
There is everyreason to believe that the types with aTrocho- 
sphere larva, viz. the Rotifera, 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 these types 
very probable. All these types, together with any other 
types which can be proved to be related to them, are des- 
cended 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 Actinotrocha' is related to the 
Echinoderm larva3 cannot be settled. Its characters may 
possibly be secondary, like those of the mesotrochal larvee 
of Chaetopods, or they may be due to its having branched off 
very early from the stock common to the whole of the forms 
above the Coelenterata. The position of Tornaria is still 
more obscure. It is difficult, in the face of the peculiar 
water-vascular vesicle with a dorsal pore, to avoid the con- 
clusion that it has some affinities with the Echinoderm 
larvae. Such affinities would seem, on the lines of specula- 
‘ It is very probable tliat Phoronis is iu no way related to the other 
Gephyrea, 
