78 MORPHOLOGICAL TYPES 



vertebral column. At their posterior ends they join, and the 

 single median excretory duct runs ventrally, behind the swim- 

 bladder to a urinary sinus. This opens to the exterior at the 

 urinary aperture. 



Genital System. — Gadus (and the higher bony fish) differs 

 from the dogfish in that the urinary and genital systems are 

 not intimately connected. The testes are elongated structures 

 suspended in the coelomic cavity on each side of the gut. They 

 do not connect with the kidney, but join one another posteriorly, 

 and send a single duct to open at the genital aperture. In the 

 female, the ovaries correspond in position to the testes. The 

 remarkable thing is that the ovaries are enclosed in sacs which 

 lead by a single duct to the genital aperture. Here, therefore, 

 the eggs are never shed free into the ccelom, to enter the open 

 mouths of oviducts. 



Vascular System. — The heart consists of sinus venosus, 

 single auricle, single ventricle, and bulbus arteriosus. It is 

 to be noticed that the muscular conus which was present in 

 the dogfish has disappeared, and has only left its valves as a 

 vestige. The ventral aorta gives off four pairs of afferent 

 branchial arteries, one ascending each of the first four branchial 

 arches. From these arches the blood is collected up into the 

 efferent branchial arteries which run to the lateral dorsal 

 aorta of their side. The lateral dorsal aortae are joined together 

 behind the gill-region to form the single dorsal aorta, and they 

 also join in front of the gills, so that a ring is formed called the 

 circulus cephalicus. Anteriorly the internal carotids run to 

 the head ; behind, the dorsal aorta gives off the subclavian 

 arteries to the pectoral fins, the cceliac and mesenteric arteries 

 to the viscera, and continues backwards between the kidneys 

 to the tail. 



The venous system does not differ in essentials from that of 

 the dogfish. 



Gadus is a type of one of the most successful group of 

 marine animals. It shows certain important advances over 

 the condition of the dogfish, but when compared with higher 

 forms most of its characters are seen to be specialised and 

 secondary. 



