XIII 



PHYLUM CHORDATA 



241 



the point of union of the efferent branchial arteries into a single 

 lateral trunk. There is a single abdominal pore opening on the 

 dorsal wall of the cloaca ; this leads into a cavity into which the 

 true abdominal pores, which 

 are very minute, lead. The 

 cerebral hemispheres are com- 

 pletely separated, except pos- 

 teriorly, and the dorsal part 

 of the mid-brain is not divided 

 into optic lobes. The kidney 

 (mesonephros) is elongated ; 

 it is devoid of nephrostomes 

 in the adult state. The vasa 

 deferentia, developed indepen- 

 dently of the urinary system, 

 join the unpaired terminal 

 part of the Mlillerian duct, 

 which opens into the cloaca 

 on a genital papilla. 



In accordance with these 

 differences, and others of less 

 importance, the living mem- 

 bers of the class Dipnoi are 

 divided into two orders the 

 Monopneumona, or one-lunged, 

 and the Dipneumona, or two- 

 lunged forms the former 

 comprising only Ceratodus, 

 the latter Protopterus and 

 Lepidosiren. 



The Dipnoi are* a very 

 ancient race. The genus 

 Ceratodus itself extends back 

 to the early Mesozoic, and 

 the remains of allied forms 

 {Dipterus and other genera) 

 are found in Devonian and 

 Carboniferous formations. But 

 if, as is conjectured, the 

 Arthrodira are to be regarded 

 as Dipnoi, then the group 

 dates back as far as the 

 Silurian. The evidence for this conclusion is, however, by no means 

 complete, as our knowledge of the structure of the extinct Fishes 

 in question is necessarily meagre. They had the head and anterior 

 part of both dorsal and ventral surfaces (Fig. 865) protected by 

 bony plates, the system of head-plates being connected with those 



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