. Bulletin . inous pocket, the separation is complete. In the larvalAmmocoetes, however, there exist two larval pericardio-peri-toneal foramina, one on either side. Communication betweenpericardial and peritoneal cavities exists also in Elasmo-branchs and Ganoids, but only during embryonic life in allhigher vertebrates. Although its embryological development is not yet workedout, the sinus venosus doubtless at first receives a right andleft duchis Cuvieri, like Petromyzon (Goette), and higherforms. The right ductus has probably been altogetherin a manner which will be discussed later. Th
. Bulletin . inous pocket, the separation is complete. In the larvalAmmocoetes, however, there exist two larval pericardio-peri-toneal foramina, one on either side. Communication betweenpericardial and peritoneal cavities exists also in Elasmo-branchs and Ganoids, but only during embryonic life in allhigher vertebrates. Although its embryological development is not yet workedout, the sinus venosus doubtless at first receives a right andleft duchis Cuvieri, like Petromyzon (Goette), and higherforms. The right ductus has probably been altogetherin a manner which will be discussed later. The left ductusCuvieri has apparently fused with the sinus venosus, and isrepresented by the external portion of the posterior expanded 23 36 C. M. Jackson. division of the sinus, which still receives the right anteriorand posterior cardinals. In the absence of a contractile valved conus arteriosus,Bdellostoma and all the Cyclostomata resemble the Teleostsmore than the Elasmobranchs. (See text figures A, B, C.). A, Elasmobranch. B, Ganoid. C, Teleost. J, sinus venosus. a, ventricle. c, conus arteriosus. b, bulbus , ventral aorta. k, valves.(From Boass Zoology.) In this respect the Bdellostoma is probably not primitive,since in the Sharks and Ganoids there is clearlj^ a tendencyto a reduction in the number of the valves and the size ofthe conus.* Contrary to Mullers statement (Vergl. Anat. der Myxi-noiden, pp. i8o), the bulbus aortse is 7iot entirely devoid ofmuscular fibers. As in the arteries in general, there is a smallamount of plain muscle fibers mixed with the thick laj^er ofelastic fibers. The bulbus is evidently not contractile, how-ever, in the ordinary sense of the word. *It is quite possible, however, that the absence of a conus represents a conditionmore primitive than the Elasmobranch. No valved conus appears during the devel-opment of Petromjzou. 24 Vascular System of Bdellostonia dombeyi. 37 Variations from the structure of the typical ventral
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