268 



CHORDATE ANATOMY 



dorsal aorta of the trunk region. From the dorsal aorta paired inter- 

 segmental vessels are given off to the body-wall and a series of median 

 unpaired vessels to the aUmentary canal. In the tail region the caudal 

 artery has intersegmental connexions with the caudal vein. Beginning 

 at the anal region blood from the caudal vein may be returned to the heart 

 either through the right postcardinal vein or by the subintestinal vein. 

 Like the hepatic portal vein of vertebrates, the subintestinal vein of Amphi- 

 oxus breaks up in capillaries within the liver. Anteriorly the hepatic 

 capillaries unite to form the hepatic vein which carries blood to the sinus 

 venosus and the truncus arteriosus. Amphioxus has no renal portal 

 system such as occurs in fishes and amphibians. Venous blood is brought 



SINUS VENOSUS 



SEMILUNAR* 



VALVES ""■ — 



Fig. 246. — 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.) 



from the anterior part of the body by the precardinal veins and from the 

 posterior body- wall by the postcardinal veins. These veins unite with the 

 sinus venosus by means of paired common cardinal veins or ductus 

 Cuvieri. The vascular system of Amphioxus thus resembles that of cyclos- 

 tomes and elasmobranchs. The blood, however, lacks hemoglobin and is 

 colorless. Blood cells are scarce. (Fig. 245, A) 



Cyclostomes. The vascular system of cyclostomes shows little 

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

 bers, sinus venosus, atrium, 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. 



