PARALLELISM. 193 



trachia and Reptilia the cephalad (auricular) division 

 is divided into two chambers by a septum ; while in 

 the birds and Mammalia the caudad division (ventricle) 

 is also so divided, making four chambers in all. 



The sources of the great vessels which distribute 

 the blood to the body and return it to the heart, dis- 

 play the same successional relation of types. In 

 the Acrania (amphioxus), the Marsipobranchii, and 

 most of the fishes, the vessel (truncus communis) 

 which receives the blood from the central organ, gives 

 off several branches on each side, which are distributed 

 to skeletal bars or arches which are in immediate con- 

 tact with water, which aerates the blood. They then 

 return, and, first sending the carotids anteriorly, unite 

 dorsad to the heart, and form the aorta posteriorly. In 

 the Batrachia, where aerial respiration succeeds to an 

 aquatic one during the life of the animal, the number 

 of the vessels contributing to form the aorta is re 

 duced from five to three in the sucessive types. One 

 of the arches is aborted as an arch, and sends the cir- 

 culating fluid to the modified swim-bladder of the fish, 

 or lung, where it is aerated. This aerated blood is 

 returned to the heart with non-aerated blood from 

 other organs, and the mixture is sent throughout the 

 body. In the reptiles we have essentially the same 

 system, but the aorta-origins are reduced to two, and 

 one, on each side. Next a division of the truncus com- 

 munis ensues, which corresponds functionally with 

 that in the ventricle, so that the impure blood from 

 one auricle is sent into the ventricle (right) which com- 

 municates with the lung ; and the aerated blood is 

 then returned to the other auricle, which pours its 

 contents into the left auricle, which drives it into the 

 aorta, and thus throughout the body. Thus pure or 



