16 DR. J. F. GEMMiLL ON [May 12, 



heart and vessels in this group (PI. lY. fig. 20). 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 men- 

 tioned. Auricles and ventricles are completely separate, and the 

 sinus venosi communicate only by a narrow neck. Each sinus 

 venosus has a pair of ducts of Ouvier, the inner or adjacent 

 ducts being smaller than the outer. This difference depends 

 mainly on the fact that the inner or adjacent ducts are small and 

 short. They can be traced backwards inside the substance of the 

 head-kidney, but are soon found to unite and to break up into 

 venules in the lymphoid tissue (PI. III. fig. 20, Vc'). 



The glomerulus of the head-kidney is shown in JPl. lY. fig. 31. 

 It is greatly elongated in a transverse direction, and 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 17), where one of the urinary 

 bladders in a -double monstrosity had no urinary pore. 



III. Union at Posterior Part of Bod//. 



In Class I., the twin heads lie symmetrically, side by side, 

 exhibiting lateral imion, but in Class II. there is a slight conver- 

 gence ventrad of the sagittal planes of the twin bodies ; while in 

 Class III. this convergence exists in a very marked degree, giving 

 rise to ventro-lateral union of the twin bodies, or, even, in 

 extreme cases, to what may be described as ventral union. 

 Roughly speaking, the further back union of the twin bodies 

 takes place, the greater is their ventral convergence. This is in 

 harmony with the fact that the twin bodies are lying tangentially 

 on the surface of a single small yolk-sphere. 



It will be convenient to subdivide Class III. into (a) cases in 

 lohich thnion takes 'place well in front of the vent, and (b) cases in 

 tohich union taJces place qidte close to the vent. 



In group [a) the alimentary canal is single for a considerable 

 distance posteriorly, the imited portion being provided with two 

 dorsal mesenteries, one from each twin body ; the head-kidneys 

 are quite separate and are normal, but their inner or adjacent 

 Wolffian ducts end blindly in the mesonephric region, while the 

 outer Wolffian ducts pass backwards as the ureters of the single 

 normal bladder (PL III. fig. 22). In this group, moreover, the 

 ventral convergence of the twin bodies is not too great but that 

 it can readjust itself at the region of transition, and allow the 

 spinal cords, as in Classes I. and II., to unite anterior to the 

 place of fusion of the notochords. 



Group (6) of Class III. includes cases in which union takes 



