444 MORGAN: [Vol. V. 



water pore, the development of the proboscis vesicle, and the 

 meshwork of mesenchymatous cells over the oesophagus in the 

 two forms will defy explanation as external adaptation. 



Moreover, the points of difference between the larva? come 

 into the later life of the Tornaria, and it must not be overlooked 

 that the posterior ciliated band must not only have had a later 

 origin in the larval history, but has as well an essentially different 

 structure from the longitudinal band. 



Concerning the relationship between Balanoglossus and the 

 Vertebrates, I have little new to add to what Bateson has already 

 given, nor does the subject come so much within the scope of 

 the present paper. The resemblances which Bateson pointed 

 out have been quoted on a previous page and need not appear 

 again. I have called attention to the close similarity in the 

 method by which the neural plate is formed in the Tornaria and 

 in Amphioxus. In passing, I may say it seems to me that the evi- 

 dence given by both of these forms and by the ontogeny of the 

 Vertebrates points out that ancestrally the dorsal nerve chord 

 probably arose as a single median unpaired structure, as has 

 been suggested by others, and that its later bilaterality may be 

 entirely a secondary phenomenon, so that all attempts to reduce 

 it back to the paired chords of Annelids or Nemertians may be 

 futile. 



I have studied with some care the structure of the gills of 

 Balanoglossus and their supporting chitinous bars, and have 

 satisfied myself as to the identity of the structures in Balano- 

 glossus and Amphioxus. The tongue bars growing down from 

 the dorsal side in both forms divide each gill slit into two parts. 

 Moreover, the relationship of the chitinous rods in the primary 

 and secondary (tongue) bars is identical in the two forms. To 

 find such an astonishing agreement in these details, which are 

 in all extremely complicated, can only point, I believe, to a rela- 

 tionship between Balanoglossus and Amphioxus. 



One word, before leaving the subject, as to the " notochord." 

 Whether we are justified in regarding the forward extension of 

 the dorsal wall of the gut in Balanoglossus and Cephalodiscus 

 as the homologue of the chorda dorsalis of the Vertebrates is 

 open, perhaps, to doubt. Nevertheless it seems probable that 

 they must be looked upon as part of the same process ; viz. a 

 specialization of the dorsal wall of the gut into other than diges- 



