BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA, 171 



brates arose from unsegmented worms, in which two lateral nerve- 

 cords were supposed to have coalesced dorsally instead of ventrally, as 

 in Annelida. 



Following this, one is reminded of Hubrecht's theory, which allies 

 Vertebrates with Nemertines, and sees the Vertebrate notochord 

 reflected in the Nemetine proboscis sheath. 



By no means least important is the celebrated Annelidan theory of 

 the origin of Vertebrates first originated by Dohrn x and Semper. 2 A 

 theory which, in spite of all attacks, still survives, and at present 

 seems to be more probable than any other. 



Finally, the alliance of Balanoglossus with Ascidians, Amphioxus, 

 and Vertebrates, recently advocated by Bateson, 3 must be mentioned. 

 Interesting though this is, it cannot yet be considered as sufficiently 

 established to be accepted without reserve j but if more evidence for 

 it be forthcoming, it is a moot point whether our existing notions of 

 the relations of Vertebrates and Annelida will not have to be modified, 

 for we know of no existing Annelid which has relationships with Bala- 

 noglossus. And here I would point out that my own researches on 

 the cranial nervous system and sense organs of Vertebrates, instead 

 of supporting the alliance of Balanoglossus with Vertebrates as high 

 as fishes, present rather a hindrance in the way of such alliance, whilst 

 they are still more opposed to the alliance of Vertebrates with existing 

 Annelida. 



That Vertebrates have their nearest allies, except Balanoglossus, in 

 the group of Annelida, is becoming more and more obvious from recent 

 researches, especially from those of Dohrn ; but the links of such an 

 alliance seem to have been rather in long extinct Annelida than in 

 any at present existing. 



In the following pages an account will be given of the morphology 

 and development of the branchial sense organs and associated ganglia 

 in Amphibians and Fishes, chiefly in Elasmobranchs. The branchial 

 sense organs are those sense organs which have usually been called 

 organs of the lateral line, and were formerly called "segmental sense 

 organs " by me. The name " organs of the lateral line " is bad, be- 

 cause it chiefly refers to those sense organs along the lateral line of 



1 Dohrn, ' Ursprung der Wirbelthiere,' 1875. 



2 Semper, " Verwandschaf tsbeziehungen der gegliederten Thiere," ' Arbeiten a. d. Zool. 

 Institut zu Wiirzburg,' 1875. 



3 Bateson, W., "Development of Balanoglossus," 'Quart. Journ. Micro. Sc.,' Supple- 

 ment, July 1885. 



