text:  Thus we find the heart of the mammalia which finally possesses four distinct cavities at first in the condition of a prolonged tube being a dilatation of the principal arterial trunk and resembling the dorsal vessel of the articulated classes; subsequently it becomes shortened in relation to the rest of the structure and presents a greater diameter whilst a division of its cavity into two parts--a ventricle and an auricle--is evident as in fishes; a third cavity like that possessed by reptiles is next formed by the subdivision of the auricle previously existing; and lastly a fourth chamber is produced by the growth of a partition across the ventricle; and in perfect harmony with these changes are the metamorphoses presented by the system of vessels immediately proceeding from the heart