102 ADAM SEDGWICK. 



mouth. I have pointed out that the blastopore in becoming 

 the larval mouth must have become highly specialised and 

 unable in most cases to repeat its ancestral history in the larval 

 development, and that the behaviour of the blastopore becomes 

 much more intelligible, though, I admit, not entirely so.^ The 

 remainder of my hypotheses are simply following the lines of 

 the recent speculations on the origin of the nervous and 

 muscular tissues. My speculations, like these, are based (1) 

 on facts of Coelenterate anatomy which have been mainly 

 brought to light by the magnificent work of the Hertwigs. (2) 

 On facts of embryonic development which have been for the 

 most part long known, but have recently been added to in an 

 important manner by Hatschek's work on Amphioxus and 

 B a 1 f o u r ' s discovery of the embryo of Peripatus capensis. 

 The object of my speculation has been to extend Balfour's 

 theory of the Triploblastic nervous system to the remaining 

 systems of organs ; in other words, I have attempted to show 

 that the majority of the Triploblastica (I confine 

 myself to the Annelida, Arthropoda MoUusca, Verte- 

 brata and certain small groups, e.g. Balanoglossus, 

 Sagitta, Brachiopoda) are built upon a common 

 plan; and that that plan is revealed by a careful 

 examination of the anatomy of Coelenterata : that all 

 the most important organ systems of these Triplo- 

 blastica are found in a rudimentary condition in 

 the Coelenterata; and that all the Triploblastica re- 

 ferred to must be traced back to a common diplo- 

 blastic ancestor common to them and the Coelen- 

 terata. 



* I shall return to a consideration of the behaviour of the blastopore in the 

 second part of this paper. 



