INTRODUCTION. 



(Higher vertebrata than fishes possess a still more complex arrangement (c/dasma), a 

 redistribution, at the point of decussation, of the fibres in their passage forward to the 

 orbit. The physiological bearing of these differences of structure is not unimportant, 

 but we are here concerned only with their systematic value. In no Ganoid fish are the 

 optic nerves known to decussate. It may be that a chiasma is always present, but 

 whether this be so or not, non-decussation of the optic nerves associates the Ganoids, 

 Sharks, Rays, and C/amara, and separates them from Teleostei. 



Miiller includes free gills and the presence of a gill-cover as a third distinctive 

 feature of Ganoids. In Chimara, Sharks, and Rays (hence termed Masmobranchii by 

 Bonaparte) the gills adhere to the outer wall of the gill-cavity, and are thus compara- 

 tively fixed, being attached at each end. Water enters the respiratory chamber by from 

 five to seven lateral openings in the case of the Plagiostomi ; by a single aperture 

 (leading to four branchial clefts) in Chimcera. In the last genus a rudimentary cartila- 

 ginous gill-cover occurs. Teleostei have gills supported on branchial arches, but not 

 attached to the sides of a gill-cavity. The outer wall of the respiratory chamber is 

 formed of a movable operculum or gill-cover. Thus Ganoids, with their free gills and 

 gill-cover, resemble the Teleostei, and recede from the Elasmobranchii, so far as 

 their respiratory apparatus is concerned. Ceratodus has each of its gills attached 

 externally to the wall of the gill-cavity. 



In the respiratory organs of Ganoidei traces, more or less evident, remain of 

 structures normally present in Plagiostomi and normally absent in Teleostei. These 

 are (1) the spiracle, which represents the visceral cleft between the mandible and hyoid; 

 (2) the pseudo-branchia, a gill-like, but non-aerating appendage, 1 representing the gill of 

 the mandibular arch, the "spiracular gill" of Plagiostomi; (3) the opercular gill, 

 equivalent to the pseudo-branchia of Teleostei and to the hyoid gill of Plagiostomi. The 

 variations exhibited in the branchial apparatus of the Ganoids may be summarised thus : 



Acipenser 



Polyodon 2 



Polvpterus 2 



Lepidosteus 3 , 



Scaphirhynchus, Lepidosiren, Protopterus, Ceratodus 4 

 Amia 



Pseudo- 

 hpiracle. , , . 



r brauchia. 



Opercular 

 gill. 



1 Demme ('Das Arterielle-Gefasssystem von Acipenser ruthenus,' 1860) has maintained that in the 

 Sturgeon the pseudobranchia is respiratory. 



2 In these genera the operculum receives a branchial artery, and hence Miiller has conjectured that 

 the gill may be present in an early stage of growth (' Fernere Bemerk. iiber d. Bau d. Ganoiden, Monatsb. 

 Ak. Berl., p. 71, 1846). 



3 The homology of the pseudo-branchia in this genus is not perfectly clear. 



4 Not functional in Ceratodus. 



