THE RELATIONSHIP OF THE CHELICERATE ARTERIAL SYSTEM ...

FIRSTMAN-CHELICERATE ARTERIAL SYSTEM AND ENDOSTERNITE 19^apodemal endoskeletondorsal vesse!pharynx'perineural vascular membrane'l mm coxa of leg 2abdominal nerve trunkFig. 12,-Midsagittal view of the cephalothoracic tegion of Eremobates sp., a solpugid, seen fromthe left. Note that a mesodermal endosternite is lacking. An apodemal invagination of the exoskeleton,derived from the tritosternum, forms a functional analogue of the endosternite. The centralnervous System is shown with dark shading.The Pulmonate Arachnid Orders:ScorpionidaThelyphonidaSchizomidaAmblypygidaAraneidaOrder Scorpionida—The circulatory System of scorpions has been described by variousinvestigators, including Newport (1843) in Androctonus and Buthus, Houssay (1886,1887) in Androctonus and Buthus, Schneider (1892) inButhus, Petrunkevitch (1922) inCentrurus, and Buisson (1925) in Buthus. According to these authors, the arterial Systemconforms to the general pattern that exists in other pulmonates (Fig. 13): a paired aortagives rise to a paired thoracic sinus which lies on top of the subesophageal ganglionicmass. The thoracic sinuses give rise to appendicular arteries and to a series of circumneuralarteries which surround the central nervous System. At their posterior ends, thethoracic sinuses give rise to a common, median, unpaired supraneural artery which carriesblood posteriad into the abdomen.The endosternite of the scorpion has been described by Beck (1885) in Androctonusand Buthus, by Bernard (1894c) in Palamnaeus, by Schimkewitsch (1894) in Androctonus,and by Pocock (1902) in Palamnaeus, lurus, Bothriurus, and Centruroides. 1tconsists of a pair of longitudinal rods which join each other posteriorly, where they alsojoin a transverse muscular partition, the diaphragm, which separates the cephalothoracicand abdominal cavities. The endosternite is circumneural at its posterior end, where itjoins the diaphragm; i.e., it forms a complete transverse ring around the posterior end ofthe subesophageal ganglionic mass (Fig. 14A; see also Fig. 26C).The morphology of the scorpion endosternite is neither simple nor primitive; its complexitylies partly in its involvement with the diaphragm, which Bernard (1894c) regardedäs the derivative of an ancient intersegmental septum. He regarded the scorpion dia-