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. TV. fig. 28). The mesonephros is just beginning to develop 

 in connection with then- middle and posterior parts. The urinary 

 bladder opens by a mesial pore situated just behind the vent. 



The intestinal canal is completely shut ofi" 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 my specimens, either {a) in the optic-lobe 

 region, or (fi) at the medulla oblongata. These two groups require 

 to be described separately, as the details of their anatomy difier 

 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) trabeculse cranii. The two pairs of 

 trabeculje 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 trabeculse 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 lamina? of the 

 parachordals nowhere form a complete vault. Accordingly the 



