TRIANGULAR OR HEXAMEROUS T^NIjE. 397 



radii to six. Sucli a head exhibits an increased number of hooks (in 

 T. cosnurus I have counted thirty-two, instead of twenty-eight), and 

 six suckers and longitudinal vessels, instead of the normal four. 

 Usually they are approximated in pairs, of which three are present. 

 The head, as a whole, has at the same time somewhat increased in 

 size. 



But in such cases it is not only the head which is abnormal ; even 

 the jointed body of the worm has had a corresponding share in the 

 change. The neck shows, instead of ordinary flat shape, a triangular 

 form, and this passes by the growth and further development of the 

 proglottides into a structure which can hardly be otherwise described 

 than that of a double formation, and which, in its extreme forms, is in 

 principle entirely comparable with the double monsters which are 

 sometimes to be observed in man and in other animals. The parts 

 which, in the normal condition, are developed into the right and left 

 halves of a single body, become in such a case the outer halves of 

 two bodies which lie at an angle to one another, but have a single 

 interior half in common. The size and independence varies widely 

 in individual cases — perhaps in connection with the disposition of 

 the superfluous suckers. 



Even Bremser had noted these abnormalities as double monsters, 

 and had observed the connection between them and the presence of 

 six hooks. ^ Later observers (Kiichenmeister and Leuckart) have 

 been able to do little more than confirm Bremser's report, which has 

 thus obtained general acceptance. Kiichenmeister will not consent to 

 regard them as abnormalities, but as a variety, since they are deter- 

 mined by what is really a " special law of development," and not " a 

 capricious deviation in development." ^ This is, however, illegitimate, 

 being founded on conceptions which have been long since rejected 

 in Teratology. Besides, from our present standpoint, there is no 

 fundamental distinction between abnormalities and varieties, and there- 

 fore the question as to the nature of these structures would be 

 quite irrelevant if Kiichenmeister had not also regarded the form 

 with tetramerous symmetry as a variety equivalent to that with 

 hexamerous. Quite apart from the fact that a species cannot possibly 

 consist of mere varieties different from one another, to place the two 

 forms on an equal platform is an unwarranted exaggeration, since the 

 hexamerous forms occur as a very small minority, perhaps only in 

 one case out of a hundred. 



We find these six-rayed forms in many tape-worms, especially 

 among the Cystotmnice, which have indeed furnished us with 



1 "Lebende Wiirmer," &c., p. 107. 



