Characteristics of the Vertebrata. Ixxxi 



apparatus differing in its pouched arrang-ement and posterior po- 

 sition from those of Ganoidei and Teleostei ; the absence of maxillary 

 and intermaxillary cartilages; and the presence of a raised ridge 

 along the intestine constitute. The sixth order^ that of the Pha- 

 ryngohrancliiiy are the lowest of Vertebrata; their claim to that 

 title resting indeed not upon the possession by them of a vertebral 

 column, but merely upon that of a chorda dorsalis, underlying a 

 membranous neural canal, and overlying a cavity containing the 

 organs of vegetative life. Its digestive tract appears to be formed 

 as in many Invertebrata by an invagination commencing on the 

 exterior of the germinal membrane, and it is to certain of the stages 

 of its metamorphosis that certain similarly transitory phases in the 

 life-history of certain sessile Ascidians have been stated to present 

 a strong resemblance unknown in other Invertebrata. 



The Dipnoi are represented by the Mud-fishes, Lepidoslren and 

 MMnocryptis, of the South American and West African rivers. 

 They possess, in addition to small external gills, seen elsewhere 

 amongst Fishes only in developing Elasmohranchii, and to func- 

 tional internal gills covered by an operculum, a pulmonary auricle 

 into which blood is returned from two pulmonary sacs. These sacs 

 communicate with the digestive tract by a ventrally-placed opening 

 guarded by cartilage ; they receive blood in a venous, and return it 

 in an arterialized state. The Dipnoi differ further from the Ga- 

 noidei, the piscine order with which they are most nearly allied, 

 and resemble the Amphibia in the communication of their paired 

 nasal sacs with the mouth, in the possession of three external 

 branchiae, in the internal structure of their arterial bulb, and in the 

 microscopic characters of their chorda dorsalis. The totality of 

 their organism however shows them to belong to the Sub-kingdom 

 Pisces ; the persistence of the structure last mentioned ; the absence 

 of vertebral centra; the development of cycloid scales; and, more 

 distinctively, of the system of the lateral line ; the presence of 

 clavicular, of branchiostegal, of opercular bones, of dermal spines, 

 and of a spiral intestinal valve; constituting a sum of characters 

 which justify us in referring them to that class. The order D'qmoi 

 differs from the orders Elasmohranchii and Ganoidei, to which in 

 so many points it appears to be more or less closely allied in the non- 

 heterocercal character of the tail. In this particular it resembles 

 on the one hand the lowest of the Fishes; and ou the other, 



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