DOUBLE MONSTROSITY— ANAKATADIDYMUS 23 



connective tissue sheath. The third variation is the most interesting, and it is illustrated in PI. XIX. 

 fig. 81. Here the union of the twin bodies has been so directly a ventral one, that the separation 

 of ventral blastema (referred to above in connection with the formation of the composite ventral fins) 

 has affected also the walls of the last portion of the rectum, as well as the anal pits. The two vents 

 are accordingly to be looked upon as being composite structures. As will be seen from the 

 figure just quoted, the Wolffian ducts, bladders, and urinary pores have undergone a corresponding 

 rearrangement. 



Wolffian Ducts, etc. Very great variation is found in the arrangement of the ureters, bladders, and 

 urinary pores in Classes VI. (a) and VI. (b). All my specimens have two bladders, which sometimes 

 communicate with one another and sometimes are quite separate. In practically all cases, the right 

 ureter of one twin and the left ureter of the other open into one of the bladders, while the two remain- 

 ing 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 itself of an external opening. 



Attention may be drawn to Figs. 78-81 on PI. XIX., which are reconstruction diagrams 

 illustrating the principal variations referred to above. Fig. 78 is drawn from a specimen in which 

 the bladder in connection with the inner or adjacent pair of Wolffian ducts has no urinary pore, but 

 opens into the bladder connected with the outer pair of Wolffian ducts. Fig. 79 illustrates a case 

 in which the first bladder has no opening and is enormously expanded, as are also the lower ends of 

 its ureters. Fig. 80 is from a specimen in which the bladders are separate and have urinary pores 

 which open in the mid-ventral line, one behind the other. It will be seen that, in this case, the two 

 bladders lie in the same plane and have corresponding right and left sides. But the right side of 

 BL' is in connection with a left Wolffian duct, while the left side is in connection with a right 

 Wolffian duct. Such a transposition is exceedingly rare in double monstrosities. Two further 

 variations are noted by Schmitt {216), namely an instance in which there was a single large bladder 

 bifurcating into two horns, one for each embryo, and an instance in which all four Wolffian 

 ducts opened by separate pores after each had widened out into a small bladder-like dilatation. 

 Fig. 81 of PI. XIX. is taken from the case of symmetrical ventral union already selected for special 

 reference, inasmuch as the alimentary canals are united posteriorly, but open by two vents on 

 opposite sides of the body. Two urinary pores are also present, one behind each of the vents. In 

 this case, accordingly, the vents and the urinary pores lie in a plane at right angles to the sagittal 

 planes of the twin bodies. This arrangement has many parallels in teratology, e.g. in cases 

 of ischiopagous double monstrosity in the higher animals. It preserves the natural correspondence 

 between rights and lefts in the ducts and bladders, which, as has just been seen, is inverted in the 

 case of which Fig. 80 provides an illustration. 



Caudal Vessels. Traced posteriorly, the caudal veins usually unite behind the vent or vents, and 

 then divide into two vessels which pass backwards in the new plane of symmetry mentioned above, 

 and are equal or unequal according as the type of union is purely ventral or not. The caudal arteries 

 are similarly disposed, their place of union being, however, a short distance behind that of the veins. 



CLASS VII. 

 Anakatadidymi (Doubling at Anterior and Posterior Ends). 



A typical example is illustrated in PI. I. fig. 2. Fig. 3 of the same plate shows a second 

 example, which presents the very interesting anomaly that one of the twin heads shows the 

 condition of semi-cyclopia (p. 44). 



The anakatadidymi have to be subdivided into the two following groups : ( 1 ) with union by 

 the walls of the yolk-sac only ; (2) with union by the walls of the yolk-sac and, in addition, for a 

 short stretch behind that region, by contiguous ventral structures. Examples of the second group 



