2l8 WIXTERTON C. CURTIS. 



be regarded as a cestode with an unusually long neck region. 

 The essential point of difference comes when proglottids begin 

 to differentiate at the anterior end of this neck /;/ the reverse direc- 

 tion. It must however be noted that subsequently the front part 

 of this neck region becomes the neck of the next strobilla which 

 at its posterior end differentiates from behind forward. It is thus 

 possible to regard the segment formation of C. laciniatnm as 

 derived from the ordinary type by the acquisition of a long neck 

 and subsequently its differentiation in the reverse order at the 

 front end. 



While it is clear that the proglottids of the adult are serially 

 homologous throughout the worm, we may ask ourselves whether 

 there can be found any good reason for the modification of 

 structure which is so noticeable in those of the anterior region. 

 If we consider the case of such a "young adult" specimen as 

 that shown by Fig. 14, a n, PI. V., it is clear that this modification 

 of the four posterior corners into sharp projections is not a char- 

 acteristic belonging exclusively to the proglottids which had an 

 anterior origin. In this figure if we examine a, b and c we find 

 that the area occupied by proglottids having the shape in ques- 

 tion extends considerably beyond the region occupied by the 

 proglottids which probably had an anterior origin. In such a 

 specimen we can no longer determine where the non-segmented 

 zone finally disappeared, but as has been pointed out there 

 are never many more than 60 anterior ones and hence the zone 

 must have been not far from the 6oth segment. The proglottids 

 having the shape in question are found extending even back of 

 the I loth as shown in c of Fig. 14. This means when applied to 

 such a figure as 13 of PI. V., or 23 of PI. VII., that some of the 

 forward members among the simple proglottids behind the "zone " 

 are later made over into the anterior type, and in Fig. 25, b, 

 PI. VII., we see that this may begin even before the zone is oblit- 

 erated. It is thus clear that the fact of a proglottid having an 

 "anterior" origin is not the factor which has determined the 

 peculiar shape of the segments at the front end of an adult C. 

 laciniatnm, but that the antero-posterior differentiation which 

 exists extends beyond the region of the obliterated "zone" and 

 into that of the posterior segments. The cause would then seem 



