DOUBLE MONSTROSITY— STRUCTURE, CLASSES III. AND IV. 19 



The behaviour of the neural and haemal arches and of the median muscular mass corresponds 

 to the description on pp. 13, 15. These structures, however, may be studied to greater advantage in 

 the type at present under consideration, as the whole transitional region is open for observation. 

 The ventral ends of the two coraco-scapular bars fail by a wide interval to meet each other below 

 the pericardium. 



Alimentary Canal. The gullets and stomachs are separate, but union is found to take place (in 

 the specimens sectioned) at the very commencement of the small intestine. The liver forms a single 

 mass transversely drawn out and showing, on either side, the ramifications of separate bile ducts. 

 The two duets unite just prior to opening by a single aperture into the small intestine. 



Heart and Vessels. Much interest centres round the heart and blood-vessels. A recon- 

 struction drawing of the heart and origins of the vesselu of a typical specimen of this group 

 is given in PL XIX. fig. 75. The whole of this double heart lies inside a large composite peri- 

 cardial cavity, which is prolonged a little forwards on either side round the origin of the ventral 

 aorta. The ventricles are separate, the auricles communicate with one another, and there is a single 

 large sinus venosus opening by a wide ostium into the auricles at their junction. The sinus venosus 

 receives blood («) on either side from the duct of Cuvier formed by union of the outer cardinal and 

 internal jugular veins of the twin embryos (i.e. from the right duct of Cuvier of the right embryo 

 and from the left duct of Cuvier of the left embryo) ; (b) from two separate middle jugular veins ; 

 and (c) from a large trunk formed by union of the inner or adjacent internal jugular veins of the 

 twin heads. This last trunk obviously corresponds to fused adjacent ducts of Cuvier which receive 

 anterior cardinal veins only and have no corresponding posterior cardinals. The hepatic veins also 

 enter the sinus venosus, from below, but are not shown in detail here, their relations having been 

 obscured owing to injury to the liver during removal of the yolk sac. 



Kidneys. The head-kidney of a specimen belonging to this class is illustrated in PI. XX. fig. 87. 

 It resembles the type described under Class II. (p. 18). 



CLASS IV. 



Union in Pectoral Region, the Inner or Adjacent Pectoral Fins being present but united, and 



reduced in size. 1 



Parts completely doubled. Doubling extends so far back that the two heads and the gill arch 

 regions are entirely separate. 



Ventral Convergence. The tendency to ventral convergence of the sagittal planes of the twin 

 bodies now manifests itself somewhat strongly, and leads to slightly earlier union on the part 

 of certain ventral than of dorsal structures. Thus the dorsal fin, while sometimes single along its 

 whole length, is in other instances doubled anteriorly, although the ventral structures are all single 

 at a corresponding level. The pelvic fins always form a single pair. 



Notochords. The degree of ventral convergence is not so great that it cannot become rectified 

 in the posterior body region, which, accordingly, exhibits a normal bilateral structure. This condition 

 is finally established at, or near, the 36 th body segment. Neural and haemal arch cartilages are 

 disposed as already described for Class I. (p. 13). 



Spinal Cords. Anteriorly the spinal cords are further apart than the notochords, but at the 

 transitional region they rapidly approach each other and finally succeed in uniting, two or three 

 segments in front of the notochords. The united part is at first composite, and shows the 

 characteristic features already described (p. 17). 



Inner or Adjacent Pectoral Fins. In typical specimens belonging to this class, fusion of the 

 pectoral limb-cartilages tends to be more complete towards the posterior, than towards the anterior, 

 border of the fins. Thus, in the specimen from which PI. XVIII. fig. 66 is taken, near the anterior 

 border there is only a small bridge of cartilage between the fins ; further back the connecting 

 bridge is broader but shorter; while at the posterior border the limb cartilages are united along 



1 See footnote on p. 1 1. 



