6 SIRENOID AND CROSSOPTERYGIAN GANOIDS. 



Lastly, Miiller gives the abdominal position of the ventral fins as an easy external 

 mark of the Ganoid order. This is plainly not of great importance ; it is the charac- 

 teristic of one of Cuvier's sections of Teleostei or Osseous fishes, the Malacopterygii 

 Abdominales. Moreover, since 1844, Calamoichthys has been brought to light, a Ganoid 

 with no ventral fins at all. Miiller seems to have contemplated such a contingency, as 

 we see from the words : " Den Character von den abdominalen Bauchflossen halte ich 

 bloss zeitweilig fur bindend. 5 ' 



Such are the characters of the recent Ganoids included in the order by Miiller. It 

 must be added that, like the Elasmobranchii, they have a spiral valve in the intestine. 

 This structure, the function of which is imperfectly known, may be described as an open 

 spiral riband or ridge, very variable in its proportions, which lines the lower part of the 

 intestinal tube, and in many cases its whole length. Miiller speaks of it as not known 

 in all Ganoids; Lepidosteus being an exception so far as he knew. Subsequent dissec- 

 tions have demonstrated the presence of a rudimentary, but strictly homologous 

 structure in this genus also, 1 so that we may now regard the spiral valve as a fresh 

 character of Ganoids, distinguishing them from all Teleostei, associating them more 

 closely with Elasmobranchii. 



The swim-bladder, whose physiological interest is so great in certain Ganoid fishes, 

 yields also useful zoological characters. It is always present in this order, and always 

 communicates by means of a ductus pneumaticus with the alimentary canal. It is single 

 in most, but in Lepidosteus and Amia it exhibits an imperfect median septum or fold, 

 while it is double in Polypterus, Protopterus, and Lepidosiren. In Ceratodus, Lepidosiren, 

 Protopterus, Lepidosteus, and Amia the swim-bladder is more or less cellular, an 

 interesting point of resemblance to the higher vertebrates, shared by some Teleostei 

 (C/tirocentrus, Platy stoma). In Polypterus and Acipenser, probably in others, it is 

 lined with ciliated'; epithelium. 3 The ductus pneumaticus opens usually, as in most of 

 those Teleostean fishes which possess such a structure, on the dorsal surface of the 

 oesophagus ; it opens ventrally in Lepidosiren, Protopterus, Ceratodus, and Polypterus, 

 while in Acipenser it/communicates (as in the Herring) with the dorsal surface of the 

 cardiac end of the stomach. 



The essential reproductive organs (testis and ovary) of Ganoid fishes are not usually 

 continuous with the efferent ducts (vas deferens or oviduct). The sexual elements are 

 therefore, as a rule, liberated into the abdominal cavity, as in Elasmobranchs, Marsipo- 

 branchs, Ampldoxus, and certain exceptional Teleostei j [Muraenidce, Sahnonida). 

 Lepidosteus, however, agrees with ordinary Teleostei in having the reproductive bodies 

 continuous with the efferent ducts. The vasa deferentia and oviducts are not perfectly 

 separate from the ureters, as in Teleostei, but are connected, in most Ganoids, by the 



1 Noticed in Muller's " Nachschrift," ' Abh. Berlin Akad. Wiss.,' 1844, p. 91. In Amia the valve is 

 also imperfectly developed. 



2 Leydig, ' Zeits. fiir Wissensch. Zoologie,' vol. v, p. 64 (1854). 



