PETROMYZON 31 



the branchial duct, and gives off paired afferent branchial 

 vessels to the gill-arches, where they break down into the 

 capillaries of the gills. Paired efferent branchial vessels then 

 gather up the oxygenated blood and lead it to the dorsal aorta, 

 which runs back just beneath the notochord, and is continued 

 forwards into the head as the internal carotid artery. 



On each side of the dorsal aorta are paired anterior and 

 posterior cardinal veins which lie of course in the body-wall 

 or somatopleur. At the level of the sinus venosus, the 

 cardinals of each side communicate with the heart by means 

 of the ductus Cuvieri. But as the cardinals are in the body- 

 wall and the sinus venosus is in the gut- wall, the ductus 

 Cuvieri have to cross the ccelom. This they do by means of a 

 bridge of ccelomic epithelium called the transverse septum. 

 The coelom is thus divided into an anterior region surrounding 

 the heart : the pericardium ; and a posterior perivisceral 

 splanchnocoel. This division is incomplete in the Ammo- 

 ccete larva, but complete in the adult. In the adult there 

 are peculiar median ventral veins in connexion with the 

 tongue, and the ductus Cuvieri on the left side disappears. 



The blood is red owing to the presence of haemoglobin in 

 corpuscles. There is no spleen. 



Excretory System. — No nephridia are found in any Craniate. 

 The excretory organ is derived from the segmented nephro- 

 tomes, between the myotomes and the lateral plate. Typically, 

 each nephrotome contains a cavity, the nephrocoel, which opens 

 into the splanchnocoel by ciliated funnels (ccelomostomes), one 

 in each segment. The nephroccels swell out into little cavities 

 known as Bowman's capsules, into each of which a glomerulus 

 projects. Each glomerulus is formed from an arteriole from 

 the dorsal aorta and a venule leading to the posterior cardinal 

 vein of its side. Glomerulus and Bowman's capsule together 

 form what is known as a Malpighian corpuscle. From each 

 capsule, a tubule grows backwards and into the tubule of its 

 next posterior neighbour. A collecting duct is thus formed 

 on each side, and it grows back, meets its fellow in the middle 

 line, and opens behind the anus on a small papilla. This is 

 the typical structure of the vertebrate kidney, and it is to be 



