X LUNG-FISH 383 



brought back to the heart by the pulmonary vein — ^which opens to the 

 left of the partition — arrives in the left auricle. The ventricular portion 

 of the heart is also divided into a right and a left chamber by a vertical 

 partition (s.Y) which is simply a continuation of the atrial or auricular 

 septum. This also is not quite complete, an open space remaining at 

 its anterior edge through which the right and left ventricles remain in 

 continuity. The blood from the left auricle when it contracts passes 

 down the left side of this septum into the left ventricle, that from the 

 right auricle passes into the right ventricle. 



The splitting of the conus is in the Lung-fishes in an incipient condi- 



dC. 



■■■ar 



Fig. 166. 



Heart of Lspidosiren with the right side removed. (After J. Robertson.) AV.^, atrio-ventricular 

 plug ; c.w, coronary vein (cut) ; d.C, ducts of Cuvier ; p.v, pulmonary vein ; p.W.c, posterior vena 

 cava at its opening into the sinus venosus ; s.A, atrial septum ; s V, ventricular septum ; 

 s,y, sinus venosus (its opening into the right auricle indicated by an arrovv^ ; sp, spiral valve ; 

 III, VI, aortic arches cut near their ventral ends. 



tion and gives valuable clues as to the manner in which this splitting 

 has come about during the evolution of vertebrates. It will be remem- 

 bered that in Elasmobranch fishes the conus arteriosus contains pocket- 

 valves arranged in longitudinal rows, each row arising from an at first 

 continuous longitudinal ridge which bulges into the cavity of the conus 

 and later segments up into the separate valves. In Ceratodus the conus 

 similarly contains longitudinal rows of valves, but in one of these the 

 individual valves are greatly enlarged and form solid blocks in con- 

 tinuity with one another, so as to constitute a large and prominent 

 ridge projecting into the cavity. In other words this row of valves 

 besides having become exaggerated in size may be said to have reverted 

 towards the original condition of a continuous ridge. In Lepidosiren an 



