20 DOUBLE MONSTROSITY— STRUCTURE, CLASSES IV. AND V. 



their whole length. As regards the inner or adjacent coraco-scapular bars, they are quite separate, 

 except at their ventral ends, which join together and project downwards into a septum between 

 the two pericardial sacs. The ventral ends of the two outer coraco-scapular bars are very widely 

 distant from one another. 



Heart and Vessels. The same specimen may serve to illustrate the typical arrangement of the 

 heart and vessels in this group. 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 receives 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 posterior cardinals are small and short. They can be traced 

 backwards inside the substance of the head-kiduey, but are soon found to unite and to break up into 

 venules in the lymphoid tissue (PI. XIX. fig. 74). 



Kidneys. The glomerulus of the head-kidney is shown in PL XX. fig. 86. It is greatly elongated 

 in a transverse direction. 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 21), 

 where one of the urinary bladders in a double monstrosity has no urinary pore. 



Alimentary Canals. The gullets, stomachs, and first portions of intestine are separate, union 

 taking place a short distance beyond the duodenum. There are two bile ducts and livers, the latter, 

 however, being confluent on their adjacent sides. The dorsal mesentery of the intestine remains 

 double for a considerable distance after the intestinal canals have united. A mesial triangular pocket 

 or body-cavity thus lies dorsal to the first single portion of intestine. 



CLASS V. 



Union by the Body or Tail, the United Portion ending, as normally, in a Single 



Symmetrical Tail. 



PI. II. fig. 9 (external appearance, one of the components reduced); XVII. fig. 63 (notochords, 

 etc.); XIX. fig. 77 (Wolffian ducts, etc.). 



Ventral Convergence. At the beginning of the description of Class IV., ventral convergence of the 

 sagittal planes of the twin bodies was referred to as a factor interfering with simple lateral union at 

 the region of transition. In the class at present under consideration this factor is better marked 

 and operates in a higher degree. Instead of simple lateral union we have a condition which is 

 typically found in Schmitt's 1 group D (union half lateral and half ventral) (p. 3), but which is 

 exhibited also by those examples of his groups C (union chiefly ventral but partly lateral) and E 

 (union chiefly lateral but partly ventral) that approach nearest to the border line between their 

 own groups and group D. 



Eoughly speaking, the degree of interference with bilateral symmetry is to be measured by the 

 distance backwards at which union takes place, the difficulties in the way of simple adjustment being 

 greatest where union is longest deferred. This will be seen to be only natural if one remembers 

 that, to begin with, the twin embryonic axes appear tangentially on the surface of the yolk sphere 

 and are situated far apart from one another on the margin of the same blastoderm (pp. 6-7). My 

 classification is based simply on the final result, i.e. on whether the ventral convergence ultimately 

 becomes rectified or not. In Class V. rectification does take place and is so complete that, ventrally, 

 only one pair of pelvic fins occurs and the anal fin is single along its whole length, while dorsally the 

 adipose fin is single and the spinal cords complete their union almost as early as the notochords. 

 The dorsal fin, however, is partly or completely double. The level at which the notochord becomes 



1 Schmitt's paper {'216) describes carefully and at length the structure of examples of Classes V. and VI., and may with 

 advantage be referred to, for supplementary details. 



