16 SIRENOID AND CROSSOPTERYGIAN GANOIDS. 



liarities of these curious fishes seem to depend largely upon their exceptional vital con- 

 ditions. During the torpid state respiration is carried on by the swirn-bladders, which 

 temporarily discharge the function of lungs, receiving imperfectly venous, and returning 

 arterial blood. When the rising of the water permits the fish to resume aquatic habits, 

 the gills oxygenate the blood, and the swim-bladder receives arterial blood for nutrition 

 only. Lepidosiren paradoxa is similarly adapted for both atmospheric and aquatic 

 respiration. 



The discovery of Lepidosiren and its African ally Protoplerus has been in every way 

 most important. The homology of the swim-bladder of the Fish with the lung of the 

 Reptile, maintained long before by Harvey and Hunter, became clear enough now that 

 the important connecting link was supplied of a Fish's swim-bladder receiving venous and 

 returning arterial blood. The definition of the class Pisces became more comprehensive, 

 and their affinities to the higher vertebrata more apparent. From this time zoologists 

 began to remark the many points of resemblance between Fishes and Amphibia (which 

 had been commonly placed among Reptiles). Hitherto the appearance of lungs in the 

 adult had marked in the systems of naturalists the passage from the lower to the higher 

 division of cold-blooded vertebrates. The absence of gills in all stages was henceforth the 

 distinctive character of the higher groups, and zoologists began to associate the Fishes 

 with the Amphibia, and the Reptiles with the Birds, a distribution which has since been 

 fortified by many arguments drawn from osteology and development. 



The peculiarities of Lepidosiren and Protoplerus seemed so striking that when their 

 place among Fishes was conceded they ranked as a separate order (Dipnoi of Miiller). 1 

 Attempts were made to connect them with other orders of Fishes with various success. 

 Some systematists were perplexed by the affinities of Lepidosiren to several types of 

 piscine structure, as well as to the higher class of Amphibia. Professor Owen appears 

 to have seen fully their relationship to the Ganoids. The following sagacious and com- 

 prehensive sentence illustrates his views. 



" It is extremely interesting to find the Ganoid Polypterus, which of all osseous 

 fishes most closely resembles the Lepidosiren in its spiral intestinal valve, in the bipar- 

 tition of the long air-bladder, the origin of the arteries of that part, and the place and 

 laryngeal mode of communication of the short and wide air-duct or windpipe, also 

 presenting the closest agreement with the Lepidosiren in the important character of the 

 form of the brain." 2 



Not only was the general affinity of the Dipnoi to Ganoidei thus clearly anticipated, 

 but fairly successful attempts were made to indicate their nearest allies in that order. 

 In 1839 Professor Owen 3 described the teeth of Protopterus as resembling "in their 

 paucity, relative size, and mode of fixation to the maxillae, those of the C/iimesra, and 



1 • Abhandlungen der Akademie der Wissenschaften zu Bsrlin' (1844). 



2 ' Comp. Anatomy of Vertebrates,' i, p. 499 (1866). 



3 ' Linnean Transactions,' vol. xviii, pt. ii, p. 331 (1839). 



