92 VERTEBRATES WITHOUT JAWS iv. 7- 



between the gill pouches, to which it sends a series of eight afferent 

 branchial arteries. These break up into capillaries in the gills, and 

 efferent branchial arteries collect to a pair of dorsal aortae, running 

 backwards, which join and form the main dorsal aorta. This passes 

 down the trunk and carries blood to all the parts of the body by means 

 of series of segmental arteries and special vessels to the gut, gonads, 

 and excretory organs. A curious feature is that many of these arteries 

 are provided with valves at the point at which they leave the main 



trunks (Fig. 57). It may be significant 

 that such valves are not found where 

 the efferent branchials join the dorsal 

 aorta, nor at the points of exit of the 

 renal arteries, so that perhaps the 

 valves serve to reduce the pressure in 

 the majority of the arteries, while 

 leaving it high in those to the kidneys. 

 The removal of large quantities of 

 water is an important problem in all 

 freshwater animals and is facilitated 

 by a high pressure in the kidneys. 

 This must be difficult to maintain in 

 an animal with a branchial circulation 

 and hence a double set of capillaries. 

 The venous system consists of a 

 network of sinuses, with contractile 

 venous hearts in various places. 

 There is a large caudal vein, dividing 

 where it enters the abdomen into two posterior cardinals. These 

 run forward in the dorsal wall of the coelom, collecting blood from 

 the kidneys, gonads, &c, and opening into the heart by a single 

 ductus Cuvieri on the right-hand side, this being the remains of a pair 

 found in the larva. Anterior cardinals collect blood from the front 

 part of the body, and there is also a conspicuous ventral jugular vein 

 draining venous blood from the muscles of the sucker and gill pouches. 

 Besides the veins proper there is a large system of venous sinuses, 

 especially in the head. Blood from the gut passes by a hepatic portal 

 vein through a contractile portal heart to the liver, from which hepatic 

 veins proceed to the heart. 



The blood of lampreys, like that of all vertebrates, contains the 

 respiratory pigment haemoglobin, enclosed in corpuscles, here nucle- 

 ated. This arrangement immensely increases the oxygen-carrying 



Fig. 57. Valves at the origin of 

 segmental arteries of a lamprey. 



1, notochord; 2, segmental artery; 3, 



aorta. 



(From Kukenthal, after Keibal.) 



