iv. 7 



(9i) 



7. Blood system of lampreys 



The blood vascular system is arranged on the same general plan 

 as in amphioxus but there is a well-developed heart. This lies behind 

 the gills and can be considered as a portion of the sub-intestinal vessel, 

 folded into an S-shape and divided into three chambers. The heart 

 is suspended in a special portion of the coelom, the pericardium, 

 whose walls are supported by cartilage. In the larva the heart first 

 appears as a straight tube and owing to an abnormality of development 

 it sometimes fails to develop its S-shape. Contractions can neverthe- 



Fig. 56. Mid-gut of larval lamprey. 



ai. anterior region of intestine; bd. bile-duct; ca. coeliac artery; gb. gall-bladder; hp. hepatic 



portal vein; /. liver; oes. oesophagus; p. position of 'pancreas', containing islet tissue; 



pi. posterior intestine; y. yellow area where wall of intestine contains zymogen cells. 



(From Barrington.) 



less be seen in these abnormal hearts, passing from behind forwards 

 along the straight tube. Similarly in the normal heart contraction 

 proceeds in the chambers from behind forwards. The most posterior 

 chamber is a thin-walled sinus venosus, into which the veins pour 

 blood. This leads to an auricle (atrium), also thin-walled, lying above 

 the sinus. The atrium passes blood into the ventricle below it, a 

 thick-walled chamber, providing the main force for sending the blood 

 round the body. 



The heart receives nerve-fibres from the vagus nerve and contains 

 nerve-cells, some of which give a chromaffin reaction suggesting the 

 presence of adrenalin-like substances. Stimulation of the vagus nerve 

 produces acceleration of the heart-beat, followed by slowing. Acetyl 

 choline also accelerates the heart. In Myxine there are no nerves to 

 the heart or nerve-cells in it and acetyl choline has no effect. Both 

 hearts contain much adrenaline and similar substances but show little 

 change when adrenaline is added to a perfusate. 



Blood leaves the ventricle by a large ventral aorta, running forwards 



