THE RELATIONSHIP OF THE CHELICERATE ARTERIAL SYSTEM ...

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40 THE JOURNAL OF ARACHNOLOGYSnodgrass (1935) has described horizontal diaphragms in insects (Fig. 28) and thesehave a vascular function, for they separate blood sinuses. There are two principal horizontalsepta in insects: (1) a dorsal diaphragm, separating the pericardial and perivisceralhemocoels, and (2) a ventral diaphragm, separating the perivisceral and perineuralhemocoels. Both of these diaphragms are muscularized by transverse fibers. The ventraldiaphragm holds a particular interest because it lies in a plane which separates the intestinefrom the central nervous system, and thus it suggests the hypothetical cheliceratecondition depicted in Fig. 27D. Although I do not intend here to suggest homologiesbetween the Insecta and the Chelicerata, I think that the mere presence in insects of aheartpaired aortasupraneuralarterypedipalpa! arterycheliceral artery•cheliceral nerve trunkcircumneural arteryesophagusFig. 29.--Hypothetical arterial System of a prechelicerate, showing a paired supraneural artery. Theview is anterodorsolateral. A ghost of the central nervous system (stippled) is shown in position. SeeFig. 34A.
FIRSTMAN-CHELICERATE ARTERIAL SYSTEM AND ENDOSTERNITE 41transversely muscularized horizontal septum, lying in a plane between the gut and thenervous System, supports my hypothesis that a similar condition could have existed inprimitive chelicerates.supraneuralarteryFig. 30.-The thoracic sinus stage of arterial development. This stage persists in the adults of allpulmonate arachnids. See Fig. 34B.The Evolution of Arterial MembranesIn pycnogonids, in Limulus, and in apulmonate arachnids, the arterial System isperivisceral. From this evidence, it seems likely that the same state of being existed also inancestral chelicerates. In the light of the ontogeny of Limulus, discussed earlier, the
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THE RELATIONSHIP OF THE CHELICERATE ARTERIAL SYSTEM ...

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