1911] Watson: The Genus Gyrocotyle. 423 



which point toward the acetabulum; and also on the fact that 

 the worm is always attached to the rosette extremity, since 

 "cestodes always attach by the head end." Haswell (1902) 

 rejects Lonnberg's and Spencer's view on orientation on the 

 basis of homologies in position between the reproductive organs 

 of Gyrocotyle and of merozoic cestodes, to which in his opinion 

 it is very closely related. "The end which bears the sucker is 

 seen as the result of such a comparison, to correspond to the 

 scolex end in the segmented cestode." This homology I regard 

 as unjustifiable, as will be pointed out later. Benham (1891), 

 comparing the reproductive organs of Gyrocotyle with those of 

 Amphilina and the heterocotylean trematodes, concludes that the 

 acetabulum of Gyrocotyle corresponds to the anterior sucker of 

 the trematodes, while the rosette organ and its peculiar proboscis 

 possibly represents the posterior caudal disc of the latter class. 

 An examination of the literature of the genus thus shows 

 that the orientation of Gyrocotyle has been made on the follow- 

 ing grounds : 



1. Behavior of living animal. 



2. Cephalization of the nervous system. 



3. Homologies of the reproductive organs with similar struc- 

 ture in the merozoic cestodes and in the trematodes. 



4. Direction of spines. 



Conclusions based on a consistent and constant functional 

 orientation of the living animal in locomotion and general move- 

 ments are unquestionably well grounded. Conclusions resting 

 on the cephalization of the nervous system assume that the 

 nervous system will be centralized and most richly developed in 

 the head region. This is true for worms in general and for all 

 platyhelminths which retain in any marked degree the power of 

 moving from place to place. In the trematodes there, takes 

 place, however, a remarkable development of ring-commissures 

 in connection with the development of powerful organs of attach- 

 ment. This is especially noticeable in the large posterior termi- 

 nal sucker of the heterocotylean trematode (pi. 47, fig. 79). It 

 seems very probable that in a permanently attached form, like 

 the cestode, in which the most powerful and highly specialized 

 musculature of the body is centered in the organ of attachment, 



