172 JOHN BEARD. 



the trunk, which morphologically form only a small portion of the 

 sense organs. I have myself seen reason to reject the name segmental 

 sense organs, because although originally they are segmental, and in 

 later life may occur one in each segment of the trunk, still at first they 

 are confined to one region only of the body, the gill-bearing region, and 

 only extend into the trunk much later. Originally they are seated 

 one above each gill-cleft or over the site of each cleft, and may, there- 

 fore, be called branchial sense organs. 1 



The so-called ganglia of the posterior roots of the cranial nerves 

 arise in connection with them, and must be regarded as originally 

 special ganglia of these sense organs. 2 



One general conclusion may be referred to here, and that is, that 

 at present we are acquainted with no invertebrate nervous system which 

 is built upon the same plan as that of Vertebrates. 



The matter will be discussed later on, and I only refer to it here in 

 order that from the outset the branchial sense organs may be raised 

 from their present position of neglect and obscurity, and may be given 

 that important morphological (and physiological) place which their 

 relationships to the gill-clefts on the one hand, and to the ganglia of 

 the posterior roots of cranial nerves on the other, most certainly entitle 

 them to. 



Unlike many previous observers, I have found that it is absolutely 

 impossible to study the branchial sense organs of fishes without at the 

 same time dealing with the posterior roots of the cranial nerves, which 

 are morphologically as well as physiologically inseparably connected 

 with the former. 



It would take up too much time and space to give here a history of 

 all the researches on these two sets of organs, which have hitherto 

 been usually treated apart from each other as if they had no con- 

 nection. 



The work has been mainly carried out on embryos of Torpedo ocel- 

 lata, for which I have to thank the Zoological Station at Naples. But 

 I have also studied Teleostei and Amphibians, and have had a few 

 embryos of Mustelus and Pristiurus. However, in the descriptions in 

 the following pages, unless otherwise stated, the condition of affairs in 

 Torpedo will be understood to be under discussion. 



1 Beard, " Cranial Ganglia and Segmental Sense Organs," ' Zool. Anzeig.,' 192, 18S5 ; also 

 Froriep, "Ueber Anlagen von Sinnesorganen am Facialis, &c," 'Archiv fur Anat. and 

 Physiol.,' 1885. 



a Beard, op. cit.,; Froriep, op. cit.; and Spencer, "Notes on the Early Development of 

 Eana temporaria," ' Quart. Journ. Micro. Sc.,' Supplement, July 1885. 



