THE CIRCULATION OP THE BLOOD. 245 



cardiac physiology — one might almost say revolutionized the 

 subject. 



Owing to the limitations of our space, the references to lower 

 forms must be brief. 



We recommend the student, however, to push the subject 

 further, and especially to carry out some of the experiments to 

 which attention will be directed very shortly. 



In the lowest organisms (Infusorians) represented by Amoe- 

 ba, Vorticella, etc., there are, of course, no circulatory organs, 

 unless the pulsating vacuoles of some forms mark the crude 

 beginnings of a heart. It will be borne in mind, however, that 

 there is a constant streaming of the protoplasm itself within 

 the organism. 



The heart is first represented, as in worms, by a pulsatile 

 tube, which may, as in the earth-worm, extend throughout the 

 greater part of the length of the animal, and has usually dorsal 

 and ventral and transverse connections 



The dilatations of the transverse portions in one division 

 (metomere) of the animal seem to foreshadow the appearance of 

 auricles. 



The pulsation of the dorsal vessel in a large earth-worm is 

 easy of observation. 



In amphioxus, which is often instanced as the lowest verte- 

 brate, the blood-vessels, including the portal vein, are pulsatile, 

 while there is no distinct and separate heart. 



Although the respiratory system will be treated from the 

 comparative point of view, the student will do well to note now 

 Ab Ao -S" 



Fig. 209.— Diagram of the circulation of a Teleostean flsh (Claus). Y, ventricle; Ba, 

 bulbus arteriosus, with the arterial arches which carry the blood to the gills,; Ab, 

 arterial arches; Ao, aorta descendens, into which the epibranchial arteries passing 

 out from the gills unite; K, kidneys; 7, intestine; Pc. portal circulation. 



.(in the figures) the close relation between the organs for dis- 

 tributing and aerating the blood. 



Passing on to the vertebrates, in the lowest group, the fishes. 



