GIGANTOCYPRIS MULLERI 219 



BLOOD VESSELS 



In Doloria the heart opens obliquely into the anterior end of an aorta which lies 

 directly on the anterior end of the nerve ring (Cannon, 193 1, p. 450, fig. 4), and runs 

 backwards to a point where it bifurcates and encircles the oesophagus. This point is 

 marked by a median apodeme which runs inward from just above the frontal knob 

 through the nerve ring close against the hinder wall of the brain to attach at its inner 

 end to the hinder wall of the aorta where the latter bifurcates. In Gigantocypris the 

 same state of affairs obtains, but here the nerve ring is situated far from the heart. The 

 region between the heart and the upper end of the labrum, which marks the level of 

 the nerve ring, has become very much stretched (p. 192) to accommodate the enormous 

 nauplius eye. Hence the aorta has become correspondingly stretched to form a large 

 tube projecting, not backwards, but downwards to meet the nerve ring. 



The increase in length of the aorta can thus be accounted for, but at the same time 

 it has increased very considerably in transverse section. Similarly, the heart is relatively 

 enormous. This can be seen clearly by comparing Fig. 4 of Doloria (Cannon, 1931, 

 p. 450) with Fig. 10 of Gigantocypris, both of which show a sagittal section through heart 

 and aorta. In Doloria the section of the heart is about the same area as that through the 

 brain. In Gigantocypris it is about fourteen times the area (Figs. 10, 12). Thus the heart 

 of Gigantocypris is, relative to the nervous system, about fifty times as big as that of the 

 smaller Cypridinid. This large size is certainly emphasized by the fact that the nerve 

 ring of Gigantocypris is relatively small, but not markedly so, when it is compared with 

 such a structure as the labrum (Figs. 11,12). Even allowing for this however and making 

 a very rough comparison with the ventral body as a whole, the heart will be about twenty 

 times as big. Now the size of a heart can be taken as a direct measure of its power, and 

 why Gigantocypris should require a heart twenty times as powerful to pump its blood 

 into the body cavity is a fact for which at present I have no explanation to offer. 



In the anterior wall of the aorta lies a pair of muscles (Figs. 10, 12). These are the 

 nauplius eye muscles that I described in Doloria. In the latter form, however, they are 

 completely outside the aorta. In Gigantocypris at their lower ends they are attached to 

 the wall of the aorta, the muscle projecting into the lumen. Dorsally, above the level 

 of the aortic valve, they pass from the aorta into the pericardial floor, but their muscle 

 bodies project outwards. 



The attachments of the muscles are the same in the two forms. Dorsally they occupy 

 a bay in the attachment of the pericardial floor which I have already mentioned (p. 212). 

 Ventrally, in Doloria they attach to a point in the mid-ventral region of the nauplius 

 eye. There is a ventral connexion to the ectoderm and also paired lateral connexions 

 to the more lateral ectoderm covering the eye. In Gigantocypris these same three 

 connexions are present, but are considerably elongated owing to the enormous develop- 

 ment of the nauplius eye. The lower end of the muscles marks the level at which 

 ectodermal strands extend inwards from the sides. The median connective, however, 



is a definite apodemal intucking (Fig. 12). 



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