TUE LATERAL CANAL SYSTEM. 351 



THE LATERAL CANAL SYSTEM. 



Plate LXIX.-LXXXir.; XXXIV. figs. 1-5; XXXV. fig. 4; XXXVIII. figs. 2, 

 3, and 7 ; XXXIX. fig. 2; XLI. figs, la, 2a. 



In Volume XVII. No. 2, 1888, of the Bulletin of the Museum of Com- 

 parative Zoology the writer traced and described the " Lateral Canal 

 System " of many of the rays, sharks and chiniaMas, -with numerous illus- 

 trations, pointed out the connections between the canals of the upper and 

 those of the lower surfaces, and adopted a nomenclature which is still found 

 to be better adapted for comparisons than other systems of names more 

 recently advocated. The terms applied in that publication with slight 

 modifications are those used herein. It is not the purpose to repeat the 

 descriptions, but it may be stated in a few words that on most Selachians 

 and Chim£erans the Lateral Canal Sj'stem consists of a tube or groove, more 

 or less branching, in which nerve endings apparently of tactile functions 

 are distributed. The tubes contain mucus and communicate with the 

 water outside by means of openings rather closely corresponding in number 

 and position with the ends of the nerves within. The mucus found in the 

 tubes is no very essential part of the system, since so many forms have the 

 papilla3 in which the nerves end exposed without inclosure in a tube or 

 channel. Sometimes the tube or groove is found to have become obsolete ; 

 in such cases the ends of the nerves may appear in small isolated papillae 

 commonly in slight depressions on the skin, or they may be inclosed in 

 cysts, remnants of the tubes, as in the so-called "Vesicles of Savi" (see Lat. 

 Canal Syst. pp. 60 and 94, Plates XXXIV. and XXXV. fig. 2), where 

 they may possibly have suffered some change in function. 



That the system was primarily confined to the head is evident from the 

 course of its development in the embryo ; and that it was twofold, that is, 

 distinct on each side of the head, is sufficiently evident from the innerva- 

 tion, from the connnon lack of an aural connection across the top of the 

 head on bony fishes, and from occasional reversions to the lack of an aural 

 on various Selachians, for instances Centroscyllmm nigrmn Plate LXIX. fig. 1, 

 below, or on Heptubrunchias macnlatus, Lat. Canal Syst., Plate XIV. fig. 2. 



