MUSEUM OF COMPARATIVE ZOOLOGY. 109 



HISTORY. 



The mucous ducts and the canals weie more or less confused by the 

 earlier writers. Usually both systems were treated as apparatus for the 

 secretion of mucus, and for distributing it over the skin. It was a 

 long time after the structural differences were pointed out before the 

 difference in function was recognized. On account of the confusion, the 

 list of authors treating of the canals is made to contain also those ti'eat- 

 ing of the ducts, as there are in most instances contrasts with the 

 canals, or references to them, even in such writings as are most ex- 

 clusively devoted to the ampullse of Lorenzini. And, further, to make 

 the literature approximately complete on the embryogeny, the innerva- 

 tion, and the general homologies of the system, it is found necessary to 

 include studies of similar organs on the Fishes, the Batrachia, and the 

 Insects. Consequently a few works are cited which have indirect con- 

 nection only with the subject of this paper. 



As early as 1664 the outward openings of the ducts on the skin of 

 the Skate were noted by Stenonis. Those on one of the Sharks were 

 described by him in 1669. The information given by Blasius, in 1681, 

 was drawn from the publication of Stenonis. 



Lorenzini, 1678, in observations on the Torpedoes, recognized the ex- 

 istence of the two classes of vessels, and distinguished them by their 

 distribution and by their branchings. Following the ducts he dis- 

 covered their swollen inner terminations, now called the " ampulke 

 of Lorenzini." 



Monro, 1785, in his book on the "Structure and Physiology of 

 Fishes," figured both ducts and canals. Plate V. of his work traces the 

 canals on the head and shoulders of a Cod. Plate VI. exposes the ven- 

 tral ducts and the canals of a species of the genus Raia ; and Plate VII. 

 shows the ducts of the upper surface of the same Skate. According 

 to this author each system formed part of "a very elegant structure for 

 the preparation of the mucus." 



Geoffroy, 1802, published his opinion that the mucous ducts of 

 the Skate were the analogues of the electric apparatus of the Tor- 

 pedo. His conclusions did not meet with ready acceptance among his 

 contemporaries. 



Jacobson, 181-3, put out a short paper, entitled " Extrait d'un Mc- 

 moire sur un organe particulier des sens dans les raies et les squales," 

 in the " Xouveuu Bulletin des Sciences, par la Societe Philomatique de 



