THE RELATIONSHIP OF THE CHELICERATE ARTERIAL SYSTEM ...

FIRSTMAN-CHELICERATE ARTERIAL SYSTEM AND ENDOSTERNITE 43perineural vascular membrane»cheliceral nerve trunkFig. 32.—The definitive periganglionic stage of arterial development. This condition exists in alladult chelicerates which lack book lungs. See Fig. 34D.passes through a larval stage in which it has a paired thoracic sinus, and becauselungless spiders still possess the genetic machinery to carry their development on (inthe absence of book lungs) to the füll periganglionic sinus condition.3. Figs. 31 and 34C depict an intermediate condition between the paired thoracicsinus and the füll periganglionic sinus. This stage does not exist in any adult chelicerate,but it occurs in the larva of Limulus (Kingsley, 1893).4. The definitive periganglionic arterial sinus (Figs. 32, 34D) is enclosed by a perineuralvascular membrane which ensheathes all the nerve trunks arising from thecentral nervous system. This membrane, which consists of connective tissue, is thesubstratum for the phylogenetic development of the endosternite.