BLOOD SYSTEM 461 



Towards the end of diastole, when the pericardial dilator is fully contracted, the 

 posterior aortic-oesophageal muscles and the aortic muscles will contract. This will 

 bring the aortic tendon down on to the floor of the aorta and so squeeze the blood 

 laterally into the anterior part of the supraneural vessel (Fig. 7 D). The aorta will thus 

 be closed posteriorly and, at the same time, the contraction of the aortic muscles, since 

 these are attached further apart at their anterior ends than posteriorly, will cause the 

 anterior part of the aorta to widen and so accommodate the blood forced out at the 

 commencement of systole. 



Systole now commences, and the blood from the previous systole has reached the 

 anterior part of the supraneural artery. This blood is now forced backwards by con- 

 traction of the anterior aortic-oesophageal muscles and the aortic-endosternite muscles 

 (Fig. 7 A). From Fig. 5 it can be seen that contraction of these muscles will pull back- 

 wards the anterior faces of the laterally running part of the supraneural vessel on to 

 the posterior. Thus, just as the hinder part of the aorta was occluded, so now the 

 anterior part of the supraneural ring will be emptied and the blood passed backwards. 



When it reaches the body cavity at the hind end of the supraneural artery the valves 

 in this vessel will close. The aortic musculature now relaxes (Fig. 7 B). This opens up 

 the posterior part of the aorta. Blood cannot pass forwards into this region from the 

 body cavity because the supraneural valves are closed but, as I have already explained, 

 it will become filled with blood from the anterior end of the aorta by the downward 

 pressure of the pericardial floor. 



As the anterior end of the supraneural vessel is emptied, part of the blood, instead of 

 passing backwards, will be forced laterally into the antennal arteries. At the point in these 

 vessels where they turn sharply forwards into the bases of the antennae there is a muscular 

 system which may act as a valve, but I could not settle its structure with certainty. 



I have now described in detail what I consider to be the most probable mode of 

 action of the complicated system of muscles in the heart, pericardium and blood vessels. 

 The whole argument is based on the premise that the muscles contract synchronously 

 with the heart. On this assumption, the action of heart and pericardium which I have 

 described is, as far as I can see, the only possible method. The action of the muscles 

 of the blood vessels is simply a method which appears to me to be probable. 



The presence of a circulatory system accessory to the heart is not surprising when 

 the diameter of the blood vessels and the probable rate of heart beat are taken into 

 account. In all observed cases of Ostracods the pulse is very rapid — Miiller (1894, 

 p. 169) records 200 heart beats per minute in Cylindroleberis obloiiga — while the aorta 

 of Doloria is certainly not more than 50 /x in diameter. In these conditions the viscous 

 resistance of the blood in the capillary aorta would entirely prevent the heart beating 

 at any speed, were there not some accessory system to help the blood through the 

 vessels once it had left the heart. A similar condition has been described in Argulus 

 by Wilson (1903, p. 690), who points out that the lacunar circulation in this form cannot 

 depend on the heart alone for the movement of blood round the body, but must be 

 assisted by rhythmical contractions of the muscles of the body. 



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