THE CRANIAL NERVES AND LATERAL SENSE ORGANS OF FISHES. 187 



R. The Phtlogeny of the Lateral or Sensory Canals. 



The solution of this jjroblem directly follows ou the systematic arrangement of the 

 known facts of the anatomy and development of the lateral line system. The history of 

 our knowledge of the phylogeny of the sensory canals is coincident with three 

 discoveries — the discovery that the " mucous "' canals contained sense organs, the 

 discovery of Savi's vesicles, and the discovery of the ampulla of Lorenzini. As I 

 propose to view the evidence in a somewhat new light, and hope to indicate the various 

 stages through which the sensory canals have passed before reaching their present 

 condition, I have devoted a section to the consideration of the above question. 



Lorenzini's discovery of the existence of the ampulliform type of " mucous " organ, 

 as apart from the canal type discovered by Steno, was further augmented in 1814 by 

 Savi (178), who described (pp. 332-340) on the ventral surface of Torpedo a number of 

 sensory pits which he called the " appareil folliculaire ncrveux," and which he thought 

 w^ere innervated exclusively by the Vth nerve, and failed to find in other forms. 

 He was further unable to determine either their homology or their function. We now 

 know that all three types belong to the lateral line system, and I shall suggest that 

 they represent three stages in the development of a canal — the most superficial con- 

 dition, represented by the i)it organs and Savi's vesicles ; the full develoiDment, represented 

 by the canal ; and the intermediate type, forming neither a Savi's vesicle nor yet a canal, 

 represented by tlie ampullar of Lorenzini. 



In 1852, H. Miiller (145) stated his belief that the " mucous" canals were primarily 

 a sensory and only secondarily a mucous apparatus. He and Ley dig were the first 

 authors to rank together as sensory structures the pit organs, ampullae, and " mucous " 

 canals of Savi, Lorenzini, and Steno. McDonnell, how^ever (1860-71, 137), still con- 

 sidered them to be organs for the secretion of mucus, nor did he think that Savi's 

 vesicles belonged to the lateral system. The latter view is shared by Boll (1875, 27), 

 who believed in the independent nature of Savi's vesicles, nor did he consider them 

 connected with the electric organs as some authors had done. He states, as Savi did, 

 that they are innervated by the trigeminal nerve. Dercum (1880, 59) goes further, 

 and considers both the vesicles of Savi and the ampullae of Lorenzini to be independent 

 of the " mucous " canals. 



An important contribution to the question is supplied by Emery (1880, 66), who, 

 as far as I am aware, first discovered what undoubtedly correspond to Savi's vesicles 

 outside Torpedo (/. e. in Fierusfer). These organs are situated at the bottom of 

 epidermal pits — each pit communicating with the surface by a slit like aperture, and 

 containing only one sense organ, which corresponded in structure with the sense organs 

 in the lateral canals. They were found in regular lines on the body, and in some of 

 the lines tlie sense organs -were connected bij shalloto longltadlnal epidermal depressions. 

 These organs, besides corresponding to the vesicles of Savi, are further homologous with 

 the pit organs of Allis and tlie " Spait-papillen " of Eritsch and Ewart & Mitchell. 

 Eritsch's term was obviously suggested by the nature of the apertures of the sacs. 

 Whilst I do not mean to assert that the vesicles of Savi, the sensory depressions of 



SECOND SERIES. — ZOOLOGY, VOL. VII. 26 



