24 DOUBLE MONSTROSITY— ANAKATADIDYMUS 



are very rare. I have come across none, and must borrow the following details from a 

 description by Schmitt (216, pp. 52-3). The pre-anal edge-membranes are not united, and accord- 

 ingly a triangular gap is left immediately behind the yolk-sac. Union of the ventral edges of the 

 bodies occurs at the level of the vent, and is continued backwards throughout the first portion of 

 the anal fin. Apart from this, the two tails are quite separate, one of them overlapping the other. 

 There are two vents, close together, both looking towards the same side. Four urinary pores are 

 present, each of the Wolffian ducts having remained independent and developed a small bladder-like 

 dilatation near its posterior end. All four urinary pores open near one another just behind one of 

 the vents. In other respects this small second group exactly resembles the first or main one, in 

 which there is union by the yolk-sac only, and, accordingly, apart from the details just given, the 

 following general description applies to both. 



The twins tend to rest on their sides (PL I. figs. 2-3), the right side of one and the left side of 

 the other being for the time uppermost. Sometimes the twin bodies are so exactly opposite to one 

 another on the yolk that their sagittal planes coincide, and it then becomes a matter of indifference 

 which sides are uppermost. In such cases, on external examination, an equal area of yolk will be 

 visible in either position, and if the circulation be watched, each embryo will occasionally be found 

 to possess right and left anterior yolk-veins of approximately equal size. The yolk store disappears 

 on both sides at an equal rate, and leaves the twins united exactly opposite to one another by their 

 ventral walls. 



Sometimes, however, the embryos do not lie on exactly opposite meridians, so that their 

 sagittal planes do not quite coincide. Accordingly, a rather larger area of free yolk will be visible 

 between the twins on one side as compared with the other. They still come to rest on their sides, 

 but now those sides tend to be underneath between which the area of free yolk is the larger. 

 Towards the end of its absorption the yolk is visible on this aspect rather longer than on the 

 opposite one. 



In healthy twins the movements of breathing go on at equal rates, and it would appear that 

 these movements are usually synchronous, but this rule is by no means without exceptions. 

 The frequency of cardiac pulsation also tends to correspond in the twin hearts, but, as a matter of 

 fact, the individual beats are independent, that is neither necessarily synchronous nor alternating 

 with one another. This same point was observed by de Quatrefages in the double-headed shark 

 which he described and figured (198 PI. VIII. figs. 1, 2). 



Coste's view (4-3) that in double monster fish the hearts aid each other in maintaining an 

 active circulation by having an alternate rhythm does not apply to healthy specimens, though 

 possibly it holds good in cases where one embryo is degenerating and about to become parasitic on 

 the other. 



Over the yolk-sac, free anastomosis occurs on the part of the finer vessels belonging to the two 

 embryos. Usually also, the anterior yolk-veins are connected by one or more larger anastomotic 

 branches, but on the whole, each embryo keeps the major part of its own blood to itself. The 

 arrangement of the yolk-veins presents many variations. Each embryo tends, however, to have a 

 right and left yolk-vein, but these are seldom of equal size, and one or other of them may be 

 absent. A furrow on the anterior aspect of the yolk-sac commonly marks the boundary between 

 the two circulations. 



It was this furrow which Quatrefages and others interpreted as indicating the fusion of two 

 separate yolks. The furrow is not visible at all in very young double monsters, and is only 

 well marked during the middle stages of yolk absorption. After absorption of the yolk is 

 completed, the body cavity shows a large mesial common chamber, which is prolonged backwards 

 and forwards within each embryo, thus forming four horns. The intestinal canals are quite 

 separate, and probably also, as a rule, the livers. Union of the ventral body-walls exists from just 

 behind the breast fins to a short distance in front of the pelvic fins. The right and left abdominal 

 recti in each embryo become separated from one another as they approach the region of union, and 

 then each arches backwards, keeping to its own side of the junction line. Reaching the hinder 



