1911] Watson: The Genus Oyrocotyle. 429 



commissure disappears and the ring grows weak with the assump- 

 tion of the sessile habit and the disappearance of sense-organs; 

 while the posterior commissure develops with the increase in 

 efficiency and complexity of the organ of attachment. 



The orientation of cestodes here suggested has been advanced 

 by several investigators, on more or less substantial grounds, 

 from Perrier to the present day. This contention has been based 

 for the most part on embryological evidence, especially with 

 reference to the hexacanth onchosphaeres so characteristic of 

 cestodes. The well-established fact that the embryonic hooks 

 are at the extremity of the cysticercus opposite to the one on 

 which the organ of attachment is developed, and the further fact 

 that the hook-bearing part of the onchosphaere is directed for- 

 wards in the movement of the embryo, affords good ground for 

 seriously questioning, if not altogether- denying, the generally 

 accepted identification of the scolex as "head." Barrois (1889) 

 maintains that the anterior part of the scolex is that extremity 

 which bears the embryonic hooks; that this part of the scolex 

 gives rise to the first proglottis, which is therefore to be regarded 

 as the ' * Kopf theil ' ' of the primitive animal. Furthermore, the 

 establishment of a zone of growth in the "neck" of the strobila 

 suggests very strongly the penultimate "zone of growth" in 

 annelids, with which the "neck" of the cestode is homologized 

 if the scolex is recognized as posterior. A full presentation of the 

 evidence in favor of this orientation, derived from embryological 

 and comparative anatomical considerations such as the above, 

 was given by Cohn (1907). He remarks that the present orien- 

 tation of cestodes has been regarded as self-evident, incapable of 

 proof; and proceeds to show that, aside from the habitus of the 

 worm, there is no evidence in favor of this view. In his own 

 words: "Meine These ist, dass dem Geschlechtstiere der Ces- 

 toden ein Kopf iiberhaupt fehlt, und sein Hinterende zu einem 

 Haftorgane dem Scolex umgebildet ist." He regards the 

 hook-bearing tail-like appendage of the cysticercoids as the homo- 

 logue of the ancestral anterior extremity of the worm; this is 

 discarded, leaving the posterior organ of attachment and the 

 intermediate growing region of the body to constitute the adult 

 cestode. "Wir haben in den Proliferationsfahigen Scolices also 



