60 BULLETIN OF THE 



junction with the main tubes. But in many species, through the 

 descent of the canals below the skin, the consequent elongation of the 

 tubules, and the multitudes of branchlets by which they communicate 

 with the surface, the primary arrangement has come to be greatly ob- 

 scured. Alopias illustrates this to some extent in the Galei ; and in the 

 Batoidei instances are numerous in the Trygonidse, the Myliobatidse, 

 the Zygobatidce, and the Ceratopteridse. 



The tubes contain a thin mucilaginous liquid. This is probably for 

 the most part an excretion, and not an absolute necessity in connection 

 with the function of the system, except, it may be, in so far as it serves 

 the purpose of lubrication. Its retention is hardly possible in the open 

 grooves of various genera, on which the office of the organs is undoubt- 

 edly the same. In discussing the purpose of the liquid, one must bear 

 in mind those Teleosts in which the sense bulbs open directly on the 

 epiderm, without either groove or tube, and the likelihood that they 

 represent the primitive condition of the system from which the furrows 

 and the tubular canals have been developed. 



Absence of the mucous secretion on the skin of species well provided 

 with canals precludes consideration of the opinion that the object of the 

 latter is to cover the surface with slime. 



Series of the follicles in immediate connection with the subrostral 

 canals of certaiu species lead to the conclusion that the nerve follicles 

 of Savi are really obsolescent tubes of the canal system, in which the 

 section that forms the enclosure or follicle owes its persistence to the 

 presence of the contained nerve. In other words, the follicles repre- 

 sent vanishing and rudimentary tubes. From this it would seem as 

 if Potamotrygon, Disceus, and Urolophus, among others, may be on 

 the way to lose the canals of their ventral surfaces, as has already 

 happened in the cases of Torpedo and Narcine. 



The hyaline mucous ducts of the ampullae are unbranched, have but 

 a single aperture, are closed at the inner end, where entered by the 

 nerve, and are filled by a jelly-like mucous. Plate XXVIII. fig. 1, 

 represents a portion of those of Raia Icevis, and their distribution as 

 compared with that of the principal canals, fig. 2. 



GALEI. 



On the Sharks the canal system consists of a vessel on each side of 

 the vertebral axis, extending from the snout to the tail, connected with 

 a similar vessel on the opposite side by a transverse branch near the 



