58 MORPHOLOGICAL TYPES 



The Miillerian duct is always genital, the Wolffian duct is 

 always excretory, but in the male it is genital in function as 

 well. In some dogfish the mesonephric tubules retain their 

 funnels, opening into the splanchnoccel. 



Vascular System. — The vascular system is built on the 

 same plan as that of Petromyzon. The subintestinal vein, 

 which forms the hepatic portal vein, runs to the liver in a 

 portion of mesentery in company with the bile-duct. The veins 

 in the body-wall (or somatic veins) consist of a pair of cardinal 

 veins running parallel with and on each side of the dorsal 

 aorta. They connect with the sinus venosus of the heart by 

 the ductus Cuvieri, which cross the ccelom from the body- wall 

 to the gut- wall, in the transverse septum. The anterior 

 cardinals bring the blood back from the head (orbital sinus and 

 jugular), and from the ductus Cuvieri backwards the veins are 

 known as posterior cardinals. In addition there are paired 

 inferior jugular sinuses bringing blood back from the ventral 

 regions of the head, and paired lateral abdominal veins draining 

 the ventral posterior regions of the body- wall. All these lead 

 into the ductus Cuvieri. The hyoid sinus is in the hyoid arch. 



The heart in its pericardium is bent on itself, and is in the 

 form of an S. The sinus venosus, which receives the ductus 

 Cuvieri and the hepatic sinus from the liver, opens into the 

 auricle whence the blood passes through an opening guarded 

 by valves to the thick- walled ventricle. This lies beneath and 

 behind the auricle. In front of the ventricle is a muscular 

 conus arteriosus with two rows of valves which prevent the 

 blood from flowing back into the ventricle. The conus leads 

 through a bulbus (see p. 330) to the ventral aorta from which 

 five pairs of afferent branchial arteries are given off. These 

 break down into the capillaries of the gill-lamellae in the hyoid 

 and four branchial arches. Each set of lamellae on one wall of 

 a slit is called a demibranch. There are two demibranchs in 

 each gill-slit except the last, which has only an anterior one. 



The oxygenated blood is collected up into four efferent 

 branchial arteries which correspond to gill-slits 1 to 4. They 

 lead to the median dorsal aorta. Each efferent branchial 

 artery is made up of two collecting vessels one on each side of a 



