curious Malformation in Tsenia saginata. 117 



to the testes themselves. This is shown by the great over- 

 crowding of these organs in the median field of the segments, 

 between the two uteri. Here we have, I think, clear evidence 

 that two sets of testes, enough for two segments — fused 

 together, as it were, side by side, — overlap each other at the 

 point of junction. 



Both sets of organs in the double segments appear to be 

 functionally active, as is shown by the presence of spermatozoa 

 in the two vasa deferentia, and by the large numbers of 

 normal ova in the two uteri. 



Apart from the genital apparatus, the double segments 

 appear to be normal — the excretory system, for example, 

 shows only the usual two pairs of longitudinal vessels and 

 the usual transverse connections. 



In examining the possible causes of this malformation, it 

 must not be confused with the cases of " triradiate," " pris- 

 matic/' or " polyradiate" cestodes, which have been noted 

 by many observers, and recently studied by Foster *, who has 

 found forty-four instances of the phenomenon mentioned in 

 literature. The triradiate forms, as far as is known, are 

 always associated with a triradiate scolex, with six suckers 

 instead of four, and their segments usually have only a single 

 genital pore situated on one of the three " wings." Where 

 two or more pores are present, they are not, as in the present 

 case, placed at the same level on opposite sides, but one 

 behind the other, usually in regular alternation. It is not, 

 therefore, to cases of this sort that we must look for an 

 explanation of the double symmetrical arrangement. Had 

 the scolex been available, it is, of course, possible that the 

 worm might have proved to be some kind of " double-headed 

 monster"; but this is rendered rather improbable by the fact 

 that a certain proportion (though a very small one) of the 

 segments are of normal type, with single pore and single set 

 of genital organs. 



On the whole, it seems more likely that we have here a 

 case where the tendency repeatedly to form partially-divided 

 and " supernumerary " segments, which has been seen to be 

 very marked, has been carried a step further than usual, and 

 many of the divisions have failed to make their appearance 

 at all. We know that there is a tendency for supernumerary 

 segments to assume a position diagonal to the long axis of 

 the worm, and that they often cause the succeeding segment 

 to be pushed out laterally in the opposite direction so as to 

 form a kind of " elbow " in the strobila. If this process 



* ' Journal of Parasitology,' ii. no. 1 (1915), p. 7. 



