CLASS ELASMOBRANCHII 



339 



The body of the dogfish is covered with placoid scales (Fig. 237), 

 each of which is composed of a sharp toothlike portion projecting through 

 the skin and attached to a plate lying in the dermis. They neither 

 overlap nor touch one another. The points of these scales are directed 



Fig. 228. — Spiral valve of a ray. The arrow shows the direction in which the food 

 passes. {From Wieman, "Ge?ieral Zoology," modified from Mayer, by the courtesy of McGraw- 

 Hill Book Company, Inc.) 



backward. They thus offer no resistance to the hand if the animal is 

 stroked from the head to the tail, but when an attempt is made to stroke it 

 firmly in the opposite direction they effectively resist the movement. 



The stomach is U-shaped and is marked off from the intestine by 

 a constriction containing a sphincter muscle. The characteristic feature 

 of the digestive system of the dogfish is the presence in the intestine 



Precai^a/ vein ■ 

 Sinus \^enosu5-\ 



, Dorsal 

 I aorta 



- Cardinal ye in 



Carofid 

 artery 



^er7Ctl 



portal 

 vein 



Caudal 



artery 



Caudal 

 vein 



First gill slit 



Afferent 

 branchial artery 



Ventral aorta 



Conus 

 arteriosus - 



Subclavian 

 vein 



Intestine 

 La'i'eral vein 

 Spleen 

 He pa fic portal vein 

 Stomach 



Iliac 

 artery 



'Iliac 

 vein 



Subclavian artery 



Fig. 229. — Diagrammatic lateral view of the circulatory system of a dogfish shark. 

 Vessels carrying oxygenated, or arterial, blood in black, those carrying venous blood, light. 

 (From Woodruff, "Animal Biology," by the courtesy of The Macmillan Company.) 



of a spiral fold, the spiral valve (Fig. 228), which projects inward from 

 the wall and which, by interfering with the direct passage of the food, 

 increases the time during which it is in the intestine and therefore the 

 time allowed for absorption. 



