THE VASCULAR SYSTEM 



353 



Cyclostomes. The vascular system of cyclostomes shows little 

 advance above that of amphioxus. An S-shaped heart with three cham- 

 bers, sinus venosus, atriiim, and ventricle, is a novelty in this group. A 

 conus with valves makes its appearance at the root of the truncus arterio- 

 sus. The common cardinals drain into a thin-walled sinus venosus. In 

 the adult the left common cardinal degenerates and both precardinals 

 connect with the right common cardinal or precava as in some mammals. 

 The two posterior cardinals unite into a common cardinal which passes to 

 the left side of the body and unites with the sinus venosus. In the heart 

 atrio-ventricular valves prevent a reverse flow of blood. With the 

 development of a septum transversum the coelom becomes divided into 



SINUS VENOSUS 



SEMILUNAR) 

 VALVES 



Fig. 295. — A diagram of the primitive (fish) heart, as seen in a median longitudinal 

 section. Anterior is to the right. The course of blood in the heart — ^indicated by 

 arrows — takes the form of a letter S. (Redrawn after Keith.) 



an anterior pericardial cavity and a posterior abdominal cavity. Red 

 blood corpuscles make their appearance in this phylum and the bloodTs 

 conseq uently red. A renal portal system is wanting in cyclostomes, the 

 caudal vein draining directly into the postcardinal veins. 



Elasmobranchs. The blood vascular system of elasmobranchs differs 

 little from that of cyclostomes and with slight changes may easily be 

 derived from the latter. Associated with the appearance of paired fins 

 the subclavian and iliac arteries and veins are present. The arteries are 

 connected with the dorsal aorta. The subclavian vein is a branch of the 

 precardinal, while the iliac vein drains into the lateral abdominal vein. 

 The latter is believed to have been developed from the median ventral 

 abdominal vein of the primitive chordates. Internal jugular veins paral- 

 leling the precardinal veins are a novelty in this group. The caudal 



