THE VASCULAR SYSTEM. 525 



In tlie Dipnoi important changes in tlie heart are effected, as compared 

 with other Fishes, by the development of true lungs. Both the auricular 

 and ventricular chamber may be imperfectly divided into two, and in the 

 conus a partial longitudinal septum is developed in connection with a 

 longitudinal row of valves'. 



In Amphibia the heart is in many respects similar to that of the Dipnoi. 

 Its curvature is rather that of a screw than of" a simple S. The truncns 

 arteriosus lies to the left, and is continued into the ventricle which lies 

 ventrally and more to the right, and this again into the doi'sally ])lact'd 

 auricular section. 



After the heart has reached the piscine stage, the aui-icular section 

 (Bombinator) becomes prolonged into a right and left auricular appendage. 

 A septum next grows from the roof of the auricular portion of the heart 

 obliquely backwards and towards the left, and divides it in two cham- 

 bers ; the right one of which remains continuous with the sinus venosus, 

 while the left one is comjiletely shut off from the sinus, though it soon 

 enters into communication with the newly established pulmonary veins. 

 The truncus arteriosus* is divided into a posterior conus arteriosus (pylan- 

 gium) and an anterior hulhus (synangium). The former is provided with a 

 proximal row of valves at its ventricular end, and a distal row at its anterior 

 end near the bulbus. It is also provided with a longitudinal septum, which 

 is no doubt homologous with the septum in the conus arteriosus of the 

 Dipnoi. The bulbus is well developed in many TJrodela, but hardly exists 

 in the Anura. 



In the Amniota further changes take place in the heart, result- 

 ing in the abortion of the distal rows of valves of the conus arteriosus^, 

 and in the splitting up of the whole truncus arteriosus into three ves- 

 sels in Reptilia, and two in Birds and Mammals, each opening into 

 the ventricular section of the heart, and provided with a special set 

 of valves at its commencement. In Birds and Mammals the ventricle 

 becomes moreover completely divided into two chambers, each com- 

 municating with one of the divisions of the primitive truncus, known 

 in the higher types as the systemic and pulmonary aortae. The 

 character of the development of the heart in the Amniota will be 

 best understood from a description of what takes place in the Chick. 



In Birds the originally sti-aight heart (tig. 109) soon becomes doubled 

 up upon itself. The ventricular portion becomes placed on the ventral and 

 right side, while the auricular section is dorsal and to the left. The two 

 parts are separated from each other by a slight constriction known as the 

 canalis auricularis. Anteriorly the ventricular cavity is continued into the 

 truncus, and the venous or auricular portion of the heart is similarly con- 



1 Boas holds that the longitudinal septum is formed by the coalescence of a row of 

 longitudinal valves, but this is opposed to Lankester's statements, " On the hearts of 

 Ceratodiis, Protopterus and Chimaira," etc. Zool. Trails. Voh x. 1879. 



2 For a good description of the adult heart vide Huxley, Article " Amphibia, " in 

 the EncyclopcBdia Britannica. 



^ It is just possible that the reverse may be true, xnde note on p. 526. If however, 

 as is most probable, the statement in the text is correct, the valves at the mouth of 

 the ventricle in Teleostei are not homologous with those of the Amniota ; the former 

 being the distal row of the valves of the conus, the latter the proximal. 



