58 BULLETIN OF THE 



Potamotrygon, Narcine, Torpedo (T. calif arnica and T. marmorata) , 

 Eaia {R. ocellata), Uraptera, Syrrhina, Ehinobatus, Somniosus, Chlamy- 

 doselachus, Heptabranchias, and Cestracion (Zygsena) ; also for sketches 

 of Chimaera. Callorhynchus, and Pristiophorus ; and for the outlines 

 and sketches of such as had been drawn by Mr. Roetter. 



With a few exceptions the names of the canals are those adopted by 

 Professor Agassiz. 



The structures to which attention is here directed are those on the 

 Selachia and Holocephala, which correspond to the lateral lines of 

 the Fishes. On the individual they form a system of branching 

 canals, or tubes, tubules, and branchlets, which has received a vari- 

 ety of names at the hands of different writers : slime canal, mucous 

 tube, water canal, lateral line, etc. To avoid confusing with the 

 unbranched hyaline mucous ducts of the ampullae of Lorenzini, the 

 term canal will in this paper be applied exclusively to the organs com- 

 prised in the system under discussion. The manner in which the tubes 

 branch and connect, and the fact that they are sometimes represented by 

 mere furrows in the skin, make this designation the more appropriate. 

 In addition, since it has been pretty well established that their func- 

 tion is that of very delicate tactile organs, receiving and carrying the 

 slighter vibrations of the water, noting changes of density, currents, etc., 

 a special name, Tremognosters, to be applied to these particular canals, 

 distinguishing them from the many other canals of the body, is intro- 

 duced as likely to prove still more convenient. 



These canals, or tubes, lie in or under the skin on both body and 

 head. They open externally either as furrows or by means of pores, 

 that in some cases enter the tubes directly and in others are approached 

 by tubules. The inner layers of their walls are furnished with series of 

 nerve-endings, which, as also the external openings of tubular canals, 

 are segmentally arranged. In difierent pai-ts, the structure of the walls 

 varies from fibro-cartilaginous, on the top and sides of the skull, to 

 very delicate transparent tissue, under the snout. Granulation, appar- 

 ently resulting from calcification, occurs in the cephalic tubes of cer- 

 tain genera. There are no glandular attachments ; the vessels are 

 simply canals, open at each end, and more or less so along their sides. 

 On the head they are innervated mainly from the fifth pair, and on the 

 bodv bv a branch of the vagus, the nervus lateralis. 



The development of the system, as worked out by Balfour in Scyllium, 

 coincides closely with that of the Teleostei. According to Beard the 

 lateral line in the embryo salmon first appears opposite the hyoid arch, 



