THE CRANIAL NERVES AND LATERAL SEXSE ORGANS OF FISHES. 185 



their innervation from the glossopharyngeus. There is thus a general agreement hetween 

 Gaclus n,ndi Ilenidia, nx\A the differences hetween them are certainly not greater tlian 

 hetween Gcuhis and Amia. 



Q. The Lateral Sexse Ohgans of Vertebkatks and Invertebrates. 



Before proceeding to discuss tlie phylogeny of the lateral sense organs, it is first 

 necessary to enquire into the evidence relating to their supposed homology with the 

 lateral organs of Invertehrates, as maintained by Eisig, Whitman, and other authors. 

 The bulk of the evidence is against such an homology, hut the position of some morplio- 

 logists with regard to tlie question renders its discussion somewhat necessary. 



Leydig (1868, 126), who was one of the first authors to conclude tliat Savi's vesicles, 

 Lorenzini's ampullae, and the sensory canals all Ijelonged to the same system, /'. e. the 

 lateral line system, was also the first to connect the Ititeral sense organs with the sense 

 organs of Annelids. This view was, however, first elaborated in detail by Eisig (1879, 

 64, and 1887, 65), who in his Naples monograph devotes considerable attention to the 

 question (pp. 501-517). He endeavours to show that the two series of organs are 

 homologous in all essential respects, and even goes to the length of compai'ing a trans- 

 verse section through the branchial region of a Vertebrate with an inverted transverse 

 section of a Capitellid worm. Tlie resemblances to which be draws attention in this 

 connection are of a very superficial character, and certainly not sufficient to establish the 

 relation of the two sets of sense organs on a reliable foundation. Eisig seems to have 

 been misled by mistaking the lateralis nerve of the Lamprey for a true lateral line 

 nerve, for his theory demands that the lateral organs of Vertebrates should be metameric, 

 and the lateralis -hqvyq oi Petromyzoii, it 11 lateral line nerve, would strongly support 

 that view. We have ah-eady seen, however, that the bulk of the evidence is against the 

 primitive metamerism of the vertebrate lateral line system, and this consideration in 

 itself is sufiicient to disprove the theory that Eisig seeks to maintain. Balfour, writing 

 however, before the publication of Eisig's mature views, says (1881, 9, vol. ii. p. IIS) : — 

 " The organs which resemble those of the lateral line are the remarkable sense organs 

 found by Eisig in the Capitellida3 ; but I am not inclined to think that there is a true 

 homology between these organs and the lateral line of Vertebrata." 



In his first paper. Whitman (1881, 219) compares the segmental sease organs of 

 the Leech with the vertebrate latei-al line, and believes that both may be traced 

 back to a common origin. In his next work (1889, 220) he considerably develops this 

 view, and considers further that the lateral organs have served as a starting point not 

 only for the taste organs, nose, and ear, but also for the eye. He assumes, what is 

 indeed possible, that the vertebrate lateral organs were derived from invertebrate sense 

 organs of some sort, but it is only adding assumption to assumption to further postulate 

 a metameric arrtingement for tliese sense or<raus. The evidence is in fact whollv 

 insufficient to justify a comparison between the two series of organs, and it is hence 

 to be regretted that Dr. Whitman did not, as far as I am aware, fulfil his expressed 

 intention of publishing more fully on the question. 



