DOUBLE MONSTROSITY— STRUCTURE, CLASSES V. AND VI. 21 



normal varies somewhat, but may be put down at from the 38th to the 46th body segment. As 

 regards the caudal veins and arteries, in one particular specimen the former united five segments in 

 front of the vent, and the latter in the second pre-anal segment. 



Intestinal Canals. As a rule, the twin intestinal canals become united a considerable distance 

 (9-11 body segments) in front of the vent, the united portion being provided with two dorsal 

 mesenteries which come from below the two notochords. More rarely, union of the intestinal canals 

 takes place a few segments (6-4) in front of the vent, and I have not come across a single example 

 of Class V. in which the intestinal canals are separate along their whole length. 



Kidneys. The two head-kidneys are quite apart from one another, each being normal and 

 giving rise to a pair of Wolffian ducts. As a rule, the inner or adjacent ducts end blindly in 

 the mesonephric region, often being distended as if by the pressure of excreted fluid (PI. XIX. 

 fig. 77). In one example the inner or adjacent ducts open into a much swollen bladder which, like 

 the anterior one in PL XIX. fig. 79, is destitute of an external opening. The outer Wolffian ducts on 

 either side pass backwards as the right and left ducts of the single portion of the body, and open into a 

 urinary bladder which communicates with the exterior by the usual single opening behind the vent. 



Number. The number of double monsters belonging to this Class is relatively small. My 

 material only provided five examples, in contrast with over twenty which fell to be included in 

 Class VI. 



CLASS VI. 



Union by Posterior Part of Body, the united portion ending in a Composite Triangidar or Quad- 

 rangular Tail, in which various structures are still doubled. 



PI. II. figs. 10-12 (external appearance); VI.-VII. figs. 27-31 (transverse sections); XIX. figs. 

 7S-81 (Wolffian ducts, etc.). 



The class is characterised either by (a) very marked ventral convergence of the two sagittal 

 planes, or by (b) pure ventral union of the twins. Both of these conditions make it impossible for 

 inner or adjacent structures to disappear gradually at the region of transition, and to leave just the 

 complement of outer structures needed to make up a single bilateral region, as occurs in the earlier 

 Classes, in the manner which has already been abundantly illustrated. 



CLASS VI. (a). 



This division includes all the well-marked and typical members of Schmitt's group C (union 

 chiefly ventral but partly lateral). The posterior part of the body and the tail are characteristically 

 triangular, there being two dorsal edge membranes and a single composite ventral edge membrane 

 (PI. VI. figs. 27-29; VII. fig. 31). The latter is produced by the coming together of the outer 

 halves of twin ventral structures from which the inner halves have gradually disappeared. 



In the earlier and middle members of this division there is only a single pair of pelvic fins, 

 which is situated on the ventral aspect of the composite body and is itself composite in the sense 

 that its units represent the outer pelvic fins of the twin embryos. The vent opens behind these. 

 In the later members of the division the corresponding inner pelvic fins may be also represented, 

 forming a pair on the upper aspect of the monstrosity. They are reduced in size, and either set 

 close together or actually united with one another. 



Notochords. The notochords either unite a varying number of body segments (12 to 1) in front 

 of the rapidly thinning terminal " heterocercal " portion, or they unite in the heterocercal portion 

 itself. The difference depends on how closely the ventral convergence of the sagittal plane 

 approaches to direct ventral union, delay being naturally greatest as the latter condition is neared. 

 However, even in extreme cases, the notochords always unite an appreciable distance in front of 

 their actual termination. There thus remains a longer or shorter common pointed portion the 

 tip of which always tends to curve in the mean dorsal direction, that is away from the ventral edge 

 of the triangular tail. The caudal veins unite near the level of the vent and the caudal arteries 

 several segments further back. 



