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Show 1903.] MONSTROSITIES IN FISHES. 7 from below, and, after running alongside the optic nerve, ends in the choroidal gland of the eye. Of the two cardinal veins, the left is usually the larger. The head-kidney, or pronephros, contains a single median glomerulus of considerable size, inside the cavity of which is a large vascular tuft supplied by a branch directly from the aorta. The Wolffian ducts, on either side, begin by a funnel-shaped opening in the glomerulus cavity. Then, bending forwards, they become convoluted, and are embedded in highly vascular lymphoid tissue. They then arch backwards, remaining convoluted for a short distance, and end in the urinary bladder (PI. III. fig. 21 & PI. IV. fig. 28). The mesonephros is just beginning to develop in connection with their middle and posterior parts. The urinary bladder opens by a mesial pore situated just behind the vent. The intestinal canal is completely shut off from the yolk-mass, and there is an open diverticulum for the air-bladder. ANATOMY OF DOUBLE MONSTROSITIES. I. Union in Head-region. The anatomy of specimens belonging to this group is very complex, as the region of transition from the double to the single condition involves the brain, the cranial nerves, and the organs of special sense, as well as the cranial, mandibular, and branchial cartilages. The twin heads are placed symmetrically, side by side, and lie in the same horizontal plane. As regards the brain, union had occurred, in m y specimens, either {a) in the optic-lobe region, or {b) at the medulla oblongata. These two groups require to be described separately, as the details of their anatomy differ in many important respects. I shall begin with a typical example of {a). There are two notochords in front, and therefore, potentially at least, two pairs of parachordal cartilages, but the four cartilages have united to form a single basilar plate (PI. I. fig. 3, c). In front, the two nasal cartilages are placed widely apart; each contains a right and left olfactory pit, and is continuous behind with a pair of (now united) trabecular cranii. The two pairs of trabecular converge as they pass backwards ; their inner or adjacent elements unite to form a median flattened bar, which joins the basilar plate formed by the parachordals. At the same time, the outer elements of each pair of trabecular have diverged from the inner elements to enclose a pair of pituitary spaces, the latter lying one on each side of the median bar formed by union of the inner trabecular elements. There are only two auditory organs, and their cartilages are continuous with the outer trabecular and parachordal elements in the floor of the skull. Dorsally, the auditory capsules are connected over the cerebellum by a vault of cartilage, which is narrower antero-posteriorly than in the normal condition. Over the medulla the laminar of the parachordals nowhere form a complete vault. Accordingly the |