| OCR Text |
Show 1903.] MONSTROSITIES IN FISHES. 15 a typical specimen of this group is given in PL III. fig. 19. (The hepatic veins are not shown because, in the removal of the yolk-sac, the liver had been damaged, obscuring the relations of its vessels.) The whole of this double heart lies inside a large composite pericardial cavity, which is prolonged a little forwards on either side round the origins of the ventral aorta. The ventricles are separate, the auricles communicate with one another, and there is a single large sinus venosus opening by a wide ostium into the auricles at their junction (PL III. fig. 19, V.Au). The sinus venosus receives blood {a) on either side from the duct of Cuvier formed by union of the outer cardinal and internal jugular veins of the twin embryos {i. e. from the right duct of Cuvier of the right embryo and from the left duct of Cuvier of the left embryo); {b) from two separate middle jugular veins ; and (c) from a large trunk formed by union of the inner or adjacent internal jugular veins of the twin heads (PL III. fig. 19, Be'). This last trunk obviously corresponds to fused adjacent ducts of Cuvier which receive internal jugular veins only and have no corresponding cardinals. The head-kidney in this specimen is illustrated by PL IV. fig. 31, and corresponds to the description given on page 14. The notochords and spinal cords are still widely separate opposite the pectoral region, and they remain separate for a considerable number of somites behind it, but ultimately they fuse, so that the posterior part of the body contains a single notochord and a single spinal cord. The behaviour of the neural and harmal arches and of the median muscular mass corresponds to the description on page 8. These structures, however, may be studied to greater advantage in the type at present under consideration, as the whole transitional region is open for observation. The dorsal ends of the adjacent fifth branchial cartilages are fused, but otherwise the twin branchial skeletons are quite separate. The ventral ends of the two coraco-scapular bars fail by a wide interval to meet each other below the pericardium. In a typical specimen belonging to group {b) of Class II., in which adjacent pectoral fins were present but united and reduced in size, fusion was more complete towards the posterior (radial) border of the cartilage than towards the anterior border. Thus, near the anterior border, there was only a small bridge of cartilage between them ; further back they approached one another, and the bridge was wider ; while at the posterior border they were united along their whole length. As regards the adjacent coraco-scapular bars, they were quite separate, except at their ventral ends, which were fused together and projected downwards into a septum between the two pericardial sacs. The ventral ends of the two outer coraco-scapular bars were very widely distant from one another. The same specimen may be used as a type for illustrating the |
