THE LATERAL CANAL SYSTEM. 351 
THE LATERAL CANAL SYSTEM. 
Plate LXITX.-LXXXIV.; XXXIV. figs. 1-5; XXXV. fig. 4; XXXVIIL figs. 2, 
3,and7; XXXIX. fig. 2; XLI. figs. 1a, 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 chimeras, 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 Chimerans the Lateral Canal System consists of a tube or groove, more 
or less branching 
to) ") 
are distributed. The tubes contain mucus and communicate with the 
in which nerve endings apparently of tactile functions 
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 
papille 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 papille 
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 common 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 Centroscyllium nigrum Plate LXIX. fig. 1, 
below, or on Heptubranchias maculatus, Lat. Canal Syst., Plate XIV. fig. 2. 
