220 AVERS. [Vol. VI. 



It is only among the Ganoids and tlieir descendants the 

 Teleosts that we get the most perfect preservation of the 

 original mode of canal formation. 



Although for the most part closed, Allis has shown {loc. cit. 

 p. 491) that the lateral canal may for a part of its length remain 

 an open groove. 



Sagemehl's view, according to which the Teleost condition is 

 derived directly from the Ganoid condition as typified in Amia, 

 is, I believe, the correct view. 



Balfour states {loc. cit. p. 445, II) : " It is clear that the canal 

 of the lateral line is secondary, as compared with the open 

 groove of Chimsera or the segmentally arranged sense bulbs of 

 young Teleostii ; and it is also clear that the phylogenetic mode 

 of formation of the canal consisted in the closure of a primitively 

 open groove. The abbreviation of this process in the Elasmo- 

 branchii was probably acquired after the appearance of food- 

 yolk in the Qgg, and the consequent disappearance of the free 

 larval stage." Allis is evidently inclined to take the same view 

 of the relation of these two forms of sense-organ channels, for 

 he says (loc. cit. p. 530) : "These different conditions both in 

 the Teleosts and in the Ganoids would also be obtained if the 

 development, as shown in Amia, was simply arrested instead of 

 undergoing retrogression ; that is, by supposing that the Te- 

 leosts had never attained the Amia condition, instead of having 

 passed through it as indicated by Sagemehl." The modifica- 

 tion in the development of the canals in Elasmobranchs is 

 paralleled by the modified manner in which the auditory invo- 

 lution is formed in Amphibia and Teleosts, where the vesicle 

 does not open on to the surface of the body in any stage of its 

 growth, being formed as an involution of the nervous layer of 

 the ectoderm in the Frog and in the Teleost as a solid ingrowth of 

 this layer, in which the auditory cavity only later makes its appear- 

 ance. Here we have an ancient structure lost from the ontogenetic 

 history, and although an attempt to reproduce this structure is 

 made by the embryonic cells, they are prevented by changed 

 conditions. I think it cannot now be reasonably doubted that 

 so far as existing forms are concerned the closed canal preceded 

 all the various modifications which have been described. 



Why should the territory within the bounds of the superficial, 

 common zone of the VII and IX have developed the ear? 

 Why not some other part of the canal system .-' 



