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Show 1903.] MONSTROSITIES IX FISHES. 17 place close to the anus. Ventral convergence of the sagittal of the twin bodies is always marked, and is often extreme enough to allow the notochords to unite anteriorly to the place of union of the spinal cords and of the dorsal edge membranes. The notochords are always double opposite the vent and for a considerable distance behind it, but I have not met with any cases in which they remained separate to their extreme tips. The dorsal aortar unite at the same level as the notochords, and the caudal veins a little in advance of the aortar. On the other hand, the spinal cords, while they usually unite far back in the caudal region, in extreme cases may remain separate altogether. The ventral edge membranes tend to unite earlier than those of the dorsal edge, but in cases of pure ventral union there may be two composite ventral edge membranes at opposite corners of the quadrangular composite body, the other corners of which carry normal dorsal edge membranes. Apparently, during the concrescence of the twin bodies, the blastema for the ventral edge of each was kept apart in two halves, each of which, continuing to occupy a lateral position, met and fused with a corresponding part derived from the other twin. In all of my specimens except one (PL III. fig. 26) the vent is single, and in all of them the intestines are united close to the vent. Franz Schmitt, however {t. c. p. 53) refers to a case in which, apparently, no such union had taken place. Very great variation is found in the arrangement of the ureters, bladders, and urinary pores in this group. All my specimens have two bladders, which sometimes communicate with one another and sometimes are quite separate. In all cases, the right ureter of one twin and the left ureter of the other open into one of the bladders, while the two remaining ureters go to the second bladder. Thus, each bladder receives a right and a left ureter derived from different embryos, and, except in cases of symmetrical ventral union, the ureters which go to the one bladder may be recognised as the inner or adjacent pair, while those which go to the other may be recognised as the outer pair. In such cases, the first bladder lies anterior and ventral to the second, with which also it frequently communicates, especially when destitute of an external opening. Attention may be drawn to figs. 23, 24, 25, 26, on Plate III., which are reconstruction diagrams illustrating the principal variations referred to above. Figure 23 was drawn from a specimen in which the bladder {BL') in connection with the inner or adjacent pair of Wolffian ducts has no urinary pore, but opens into the bladder {BL) in connection with the outer pair of Wolffian ducts {WBa, WBb). Figure 24 illustrates a case in which the first bladder {BL') had no opening and was enormously expanded, as were also the lower ends of its ureters. Figure 25 is from a specimen in which the bladders {BV and BL) were separate and had urinary pores which opened in the mid-ventral line, one behind the other. It will be seen that PROC. ZOOL. Soc-1903, V O L . II. No. II. 2 |