386 THE EVOLUTION OF MAN. 



m. Third Period : Hea/rt of Cyclostoma. 



The heart dirides into two chambers, a posterior auricle (airwm) and 

 an anterior rentricle (yentricuUis). 



IV. Fowrih Period : Heart of Primitive Fishes. 

 From the anterior section of the ventricle is diflferentiated an arterial 

 stalk (hullm$ a/rteriosus) , as in all Selachii. 



V. Fifth Period : HeaH of the Mud-fishes. 



The auricle divides, bj an imperfect and interrupted partition, into 

 a right and a left half, as in Dipneasta. 



VI. Sixth Period : Heart of Atnvhihia. 



The 'partition between the right and left auricles becomes complete, a> in 

 the higher Amphibia. 



Vn. Seventh Period : Heart of Protamnia. 



The main chamber of the heart divides, by an incomplete partition, into 

 a right and a left half, as in Reptiles. 



VIII. Eighth Period : Heart oj Monotrema. 



The partition between the right and left ventricles becomes completOi as 

 in all Mammals. 



IX. Ninth Period : Hea/rt of Marsupials. 



The valves between the auricles and ventricles (atrio.ventricular valves), 

 together with the connecting filaments and papillary muscles belonging to 

 them, are differentiated from the muscular masses of Monotremes. 



X. Tenth Period : Heart of Apes. 



The main axis of the heart, lying in the central line of the bodj 

 ueoomes oblique, so th&t the' apex ia turned to the left, aa in Apeft vad 



