THE ANOBSTET OP THE CHOEDATA. 



103 



Though it is likely that many of the suggestions here made 

 may be shown hereafter to be wrong, still it has seemed well, 

 on the whole, to analyse the facts as they stood, and to endea- 

 vour to reconstruct the past stages, whose existence is indi- 

 cated by the lacunae in the sequence of these facts, avoiding 

 as far as possible a reliance upon phylogenetic changes of 

 whose occurrence we have no evidence. 



The foregoing views are, perhaps, more clearly expressed in 

 the following table, which is not meant so much as a genealo- 

 gical tree as to serve as an exhibition of the logical relation 

 of the various forms, showing their points of divergence. 



Cyclostomata. 



Protognathostomata 

 (of Balfour). 



Enteropneusta. 



? Nemertines. 



Cephalochorda. 



^Tunicata. 



Form with one gill-slit. 



