2O8 WIXTEKTON C. CURTIS. 



ably just entered the shark, the proglottids originate as follows : 

 The young worm which is recognizable by the small size and 

 the proportions of its scolex region (Fig. i, PI. IV.) at first forms 

 segments from behind forward in the typical cestode fashion (Fig. 

 2, PI. IV.). The youngest specimen I have found (Fig. I, PI. IV.) 

 appears to have something missing from the posterior end, but a 

 comparison with Fig. 2 which is a little older would indicate that 

 not more than one proglottid has been lost. The progress of this 

 early segmentation, the increase in size and the changes in the 

 proportions of the scolex parts are seen by comparing Figs. I, 

 2 and 5, which are all drawn on the same scale. The terminal 

 proglottid in such stages is invariably long as in Figs. 2 and 4, 

 but in older specimens (Fig. 7) which show no signs of having 

 lost any segments at the posterior end one is very likely to find 

 it much shorter. In Fig. 3, for example, there is a very long 

 terminal proglottid which is bent twice in places which are clearly 

 not lines of division between segments. Figs. 4 and 6 represent 

 specimens in which the most posterior line of division was much 

 fainter than the lines anterior to it. This suggested the possibility 

 that the last two proglottids in such cases had originated by the 

 division of an earlier single terminal proglottid. I have often 

 found specimens which suggested very strongly that such a divi- 

 sion of the earlier proglottids had taken place, but since it is well- 

 nigh impossible to make certain of such a process when specimens 

 can only be taken and arranged in series after being studied, I do 

 not wish to say more regarding this than that such cases as shown 

 above have frequently come to my attention and suggested the 

 possibility of the subsequent division of the elongated segments 

 found at the posterior end in very young worms. The formation 

 of simple rectangular segments from behind forward progresses 

 steadily (Figs. 6 and 7) until from 40 to 60 of these " posterior >: 

 proglottids have been developed. There then begin to appear 

 in the region just behind the scolex other proglottids. These 

 "anterior" proglottids are from the first different in shape from 

 the ones at the posterior region and this difference is, as will be 

 shown, continued into the adult. The most important fact about 

 these "anterior" proglottids is that they are differentiated from 

 in front backward. Fig. 10 shows a specimen in which a number 



