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Show 16 DR. J. F. GEMMILL OX [May 12, heart and vessels in this group (PL IV. fig. 20). There are two pericardial cavities separated by a septum of connective tissue, which is thin posteriorly, but in front is thick and contains the fused ventral ends of the adjacent coraco-scapular bars just mentioned. Auricles and ventricles are completely separate, and the sinus venosi communicate only by a narrow neck. Each sinus venosus has a pair of ducts of Cuvier, the inner or adjacent ducts being smaller than the outer. This difference depends mainly on the fact that the inner or adjacent ducts are small and short. They can be traced backwards inside the substance of the head-kidney, but are soon found to unite and to break up into venules in the lymphoid tissue (PL III. fig. 20, Vc). The glomerulus of the head-kidney is shown in PL IV. fig. 31. It is greatly elongated in a transverse direction, and the tubule from its middle compartment, representing fused adjacent Wolffian ducts, passes forwards so as to lie between the two adjacent cardinal veins which have just been referred to. It ends blindly and is so much sacculated as to suggest a certain degree of pressure in the fluid secreted by the glomerulus. A similar point will be noted later (page 17), where one of the urinary bladders in a double monstrosity had no urinary pore. III. Union at Posterior Part of Body. In Class I., the twin heads lie symmetrically, side by side, exhibiting lateral union, but in Class II. there is a slight convergence ventrad of the sagittal planes of the twin bodies ; while in Class III. this convergence exists in a very marked degree, giving rise to ventro-lateral union of the twin bodies, or, even, in extreme cases, to what may be described as ventral union. Roughly speaking, the further back union of the twin bodies takes place, the greater is their ventral convergence. This is in harmony with the fact that the twin bodies are lying tangentially on the surface of a single small yolk-sphere. It will be convenient to subdivide Class III. into (a) cases in which union takes place well in front of the vent, and (b) cases in which union takes place quite close to the vent. In group {a) the alimentary canal is single for a considerable distance posteriorly, the united portion being provided with two dorsal mesenteries, one from each twin body ; the head-kidneys are quite separate and are normal, but their inner or adjacent Wolffian ducts end blindly in the mesonephric region, while the outer Wolffian ducts pass backwards as the ureters of the single normal bladder (PL III. fig. 22). In this group, moreover, the ventral convergence of the twin bodies is not too great but that it can readjust itself at the region of transition, and allow the spinal cords, as in Classes I. and II., to unite anterior to the place of fusion of the notochords. Group {b) of Class III. includes cases in which union takes |