CLASS CHONDRICIITHYES. CARTILAGINOUS FISHES 



371 



Figure 246. Intestine of a dogfish shark cut open to show the structure of the spiral valve. 

 The arrows inside valve show the direction of movement of food. 



Circulatory system 



As in the cyclostomes and most of the 

 true fishes, the heart contains venous blood 

 only (Fig. 244). It is pumped through the 

 ventral aorta and thence into the afferent 

 branchial arteries, becoming oxygenated in 

 the capillaries of the gills. It then passes into 

 the efferent branchial arteries, which carry 

 it to the dorsal aorta. The dorsal aorta sup- 

 plies the various parts of the body. Veins 

 carry the blood back to the heart, opening 

 into the sinus venosus. Other veins, called 

 the hepatic portal system, transport the 



blood from the digestive canal, pancreas, 

 and spleen to the liver from which hepatic 

 sinuses return it to the sinus venosus. A 

 third system, the renal portal system, con- 

 veys the blood from the posterior part of the 

 body to the kidneys. Blood leaves the kid- 

 neys by way of several renal veins, emptying 

 into the posterior cardinal sinuses which 

 return it to the sinus venosus. 



Respiratory system 



Respiration is carried on by means of gills 

 (Fig. 247). These are folds of mucous mem- 



Papilla of esophagus 



Gitl slit 



Gill slif 



•Visceral arch 



Gill raker 



Visceral arch 



Pectoral girdle 



Gill ray 

 ■Gill filament 



Gill slit 



Figure 247. Respiratory structures in the dogfish shark. The left side of the phar>nx is cut 

 lengthwise and laid open to show the gills and other structures. 



