90 ZOOLOGY SECT. 



through the system : the right auricle receives impure blood from 

 the system, and passes it into the right ventricle to be pumped 

 into the lungs for aeration. Thus the four-chambered heart of 

 the higher Vertebrata is quite a different thing from that of a Fish : 

 in the latter the four chambers sinus venosus, auricle, ventricle, 

 and conus arteriosus form a single longitudinal series, whereas 

 in a Mammal, for instance, the four chambers constitute practically 

 a double heart, there being no direct communication between the 

 auricle and ventricle of the right side, or respiratory heart, and those 

 of the left side, or systemic heart. The modifications undergone by. 

 the arteries and veins in the higher Vertebrata will be best 

 considered under the various classes. 



It will be noticed that there is a sort of rough correspondence 

 between the blood-vessels of Craniata and those of the higher 



V 



V*' . 



u A m RA 



FIG. 730. Diagram of the heart A, in an Amphibian ; B, in a Crocodile. A, right auricle ; 

 A', left auricle ; Ap, pulmonary artery ; lr, pulmonary vein ; RA, aortic arches ; V, ventricle ; 

 V\ left ventricle ; V, 7, and Ve, Ve, pre- and post-cavals. (From Wiedersheim's Vertebrata.) 



Worms. The sub-intestinal vein, heart and ventral aorta together 

 form a ventral vessel, the dorsal aorta a dorsal vessel, and the 

 aortic arches commissural vessels. The heart is therefore to be 

 looked upon as a portion of an original ventral vessel, which has 

 acquired strongly muscular walls, and performs the whole function 

 of propelling the blood. There seems to be some reason for 

 thinking that the caudal, hepatic-portal, and hepatic veins 

 represent detached portions of the original ventral vessel, .while 

 the lateral veins may be compared with the lateral vessels of 

 some Annulates. 



The blood of Craniata is always red, and is specially distin- 

 guished by the fact that the haemoglobin to which it owes its 

 colour is not dissolved in the plasma as in most red-blooded Inver- 

 tebrates, but is confined to certain cells called red blood corpuscles 

 (Fig. 731), which occur floating in the plasma in addition to, and 

 in far greater numbers than, the leucocytes. They usually have 



