BLOOD SYSTEM 453 



and then passes in between the sixth and fifth (Fig. 2) and merges into the pericardial 

 floor in the region where this passes over from posterior to anterior pericardium (Figs. 

 5, 6). It now curves downwards and disappears towards the region of the hepatic 

 valves. Where it Hes in the pericardial floor it is excessively thin, and can only be seen 

 in sections when it happens to lie in the plane of the section. I have called it the lateral 

 sub-pericardial muscle. I was able to follow it clearly in polarized light in the type of 

 preparation I have already described for the study of the dorsal body wall. 



The functioning of the various muscles of the pericardium is described later (p. 456) 

 after a description of the blood vessels. 



BLOOD VESSELS 



An aorta extends backwards from the heart lying in a groove between the two halves 

 of the brain. Anteriorly it ends blindly in the parenchyma which occurs around the 

 antero-medial attachment of the pericardial floor, this latter attachment marking the 

 level of the floor of the aorta. Posteriorly it extends as far as the aortic tendon which 

 forms its roof in this region. The lower face of the pericardial dilator and the median 

 strip of the pericardial floor from this muscle, as far as the posterior limit of the aortic 

 valve, run in the middle line of the roof of the aorta. The aortic valve thus cuts obliquely 

 through the aorta (Fig. 4 A), its posterior end being in the roof and the anterior end 

 in the floor. The walls of the aorta are supported by a pair of muscles, the aortic 

 muscles (Figs. 4 A, 5, 6), which run forward from the aortic tendon and join the nauplius 

 eye muscles just below their upper attachment. Posteriorly they are flattened and form 

 part of the aortic roof, but in sections through the more anterior part they can be seen 

 lying on the inner face of the aortic walls. 



The walls of the aorta are thin but very definite and, as in all the arteries, it is im- 

 possible to distinguish any cell limits or nuclei. The aorta is supported anteriorly by 

 the parenchyma already mentioned. This extends backwards along its sides and joins 

 the gut parenchyma which runs up to the hepatic valves (Fig. 11). Apart from this 

 the aortic walls are naked. 



Liiders (1909, p. 117) refers to a vessel in Gigantocypris which must be the aorta, 

 but he describes it as a more or less indefinite split between connective tissue cells. 



At the level of the aortic tendon the aorta divides into two (Fig. 5). Each half at 

 once divides again sending one branch laterally to the antennae, and the other 

 branch posteriorly underneath the prongs of the endosternite fork and above, and 

 attached to, the lateral portions of the nerve ring. This bifurcation is probably what 

 is referred to by Miiller (1927, p. 415), when he states that the aorta sends two branches 

 to the neighbourhood of the attachment of the adductor muscle, and by Klie (1929, 

 p. 37), who describes "2 seitliche, nach der Schalenmitte gerichtete Bahnen". 



The posterior branches each give ofl^ a vessel to the mandibles and then join each 

 other behind the muscles which pass through the nervous system behind the trito- 

 cerebral commissure (Figs. 4 A, 5). At the same time they open into the body cavity 

 directly underneath the main mass of the tendon of the adductor muscle. The aorta 



3-2 



