SENSORY CANALS OF L^EMARGUS. 61 



chiefly because they are especially characterised by the ampullae at their proximal ends — 

 the ampulla? of Lorenzini. 



In Laemargus, according to the nomenclature I have adopted, there are three main 

 canals in the head region and one in the trunk — the cranial canals having over 

 four hundred sense organs, and a nearly corresponding number of tubules. The 

 ampullary canals, though numbering over one thousand, will be found to radiate from 

 four centres, all situated in the head region. Having indicated the main features of 

 the two canals, and the names by which they will be described, I need only further say, 

 before referring to the history of the subject, that sensory follicles have not yet been 

 found in Laemargus. 



The first observations relating to the lateral sense organs of fishes seem to have been 

 made by Stenonis, who, in 1664, described certain openings in the skin of the skate for 

 the discharge of mucus (4) ; and, in 1669, discovered similar openings in a shark (5). 

 About ten years later (1678), Lorenzini (6) not only found the openings described by 

 Stenonis, but made the important discovery that the openings belonged to two kinds 

 of canals (the sensory and ampullary canals mentioned above), and especially noted the 

 expansion (ampulla) at the proximal end of each of the simple (ampullary) canals. 



Since the time of Lorenzini, many anatomists and zoologists have studied the 

 " mucous " canals of fishes ; but Lorenzini's discovery that the canals were of two 

 kinds has been too often overlooked ; and it was not until 1813 that it was first suggested 

 these canals played any other part than that of secreting and distributing mucus over 

 the surface of the skin ; and not until 1868 that their right, from a morphological point 

 of view, to be considered sensory structures was finally established. 



Very little progress was made in working out the structure of the lateral sense organs 

 from the time of Lorenzini to that of Monro secundus, who* in the latter half of the 

 eighteenth century, worked at " The Structure and Physiology of Fishes " (7). Monro 

 especially directed his attention to the " mucous " canals of the skate, and he made out 

 (what seems to have escaped the notice of Lorenzini) that large nerves proceeded to the 

 gland-like masses (groups of ampullae) formed by the inner ends of what I have termed 

 the ampullary canals. There is no evidence, however, that Monro was acquainted with 

 Lorenzini's work or that he recognised the difference between the branched (sensory) and 

 simple (ampullary) canals, or specially observed the ampullae of Lorenzini ; and though 

 he traced large nerves to the central masses, he apparently considered the canals as mere 

 mucus-producing structures. He describes them as "very elegant structures for the 

 preparation of mucus for keeping moist the surface of the skin." Nevertheless, Monro's 

 noting that large nerves reached the central masses (the groups of ampullae) may have 

 helped Jacobson, in 1813, to arrive at the conclusion that the mucous canals of sharks 

 and rays were sensory organs (8). 



During the first half of the present century, the lateral sense organs attracted the 

 attention of several investigators besides Jacobson, more especially St Hilaire, 

 Treviranus, de Blainville, Della Chiaje, Savi, Mayer, and Robin. The central 



