424 University of California Publications in Zoology. I Vo1 - ,5 



there will be found in that region the greatest and most highly 

 developed mass of the nervous system, quite independent of 

 whether the attached end is the homologue of the ancestral 

 anterior or cephalic extremity or not. Therefore, I am inclined 

 to question conclusions as to orientation based on "cephaliza- 

 tion" of the nervous system, unless confirmed by other unques- 

 tioned evidence. 



The third basis on which the question has been decided, the 

 homologizing of the reproductive organs and openings with those 

 of trematodes or merozoic cestodes, can obviously only be applied 

 in the direction of trematodes as long as the orientation of 

 merozoic cestodes themselves is in question. Furthermore, 

 antero-posterior relations among these structures are fixed and 

 constant for trematodes; while among cestodes the most astonish- 

 ing variations present themselves. 



The direction of the spines is not at all a decisive piece of 

 evidence inasmuch as spines may be quite as useful to the animal 

 if directed anteriorly and serving as a means of attachment, as 

 if directed posteriorly and serving as aids to locomotion. 



Wagener's orientation of Gyrocotylr, regarding the aceta- 

 bulum as anterior, the rosette as posterior, is justified in my 

 opinion on the following grounds: 



1. This is the functional orientation of the living worm. The 

 rosette end is relatively quiescent while the acetabular end is 

 exceedingly active in exploring movements, is directed anteriorly 

 in well-defined progressive locomotion, and leads in all righting- 

 up movements. The rosette end never leads in locomotion excepl 

 when shoved backward by the doubling under of the active 

 acetabular end, and performs no movements other than a slight 

 rolling from side to side. This mode of behavior agrees with 

 that described by Wagener (1852) in his original account of 

 Gyrocotyle urna; but is totally at variance with Lonnberg's 

 (1891) observations on the same form. This discrepancy is 

 discussed above. 



2. The position of the reproductive openings in Gyrocotyle 

 as compared with that in the heterocotylean trematodes homo- 

 logizes the rosette with the posterior sucker. The birth-pore of 

 Gyrocotyle is probably, according to Goto's (1891) view of the 



