1911] Watson: The Genus Gyrocotyle. 355 



be obtained through an exhaustive study of the Cestodaria, the 

 nearest surviving relatives of the merozoic cestodes; and in this 

 possibility lies the chief interest pertaining to the former group 

 and to this investigation. 



Lonnberg (1897), in a study of the phylogeny of parasitic 

 platyhelminths, discusses the possible phylogenetic significance 

 of the Cestodaria and comes to the conclusion that they cannot 

 be regarded as ancestral forms of the merozoic cestodes : 



"Die jetzigen polyzoischen Bandwurmstrobilen sind natiirlich aus 

 monozoischen Formen hervorgegangen. Die noch jetzt existierenden 

 monozoischen Cestoden konnen aber kaum als Stammformen der poly- 

 zoischen angesehen werden. . . . Diese [Gyrocotyle and Amphilina] 

 sind wahrscheinlieh beide urspriinglieh, weichen aber in mehreren Hin- 

 sichten von den echten Cestoden recht sehr ab. Gyrocotyle ist durch das 

 Erweben von Haftorganen an beiden Korperenden, das Trichterorgan 

 am Vorderende und das Acetabulum am Hinterende, unfahig geworden, 

 sich weiter zu entwickeln. ' ' 



While the writer is convinced of the essential correctness of 

 Lonnberg ? s general contention, that the ancestors of cestodes 

 must be sought, not among Cestodaria or trematodes, but among 

 Turbellaria, it is felt that he undervalues the evolutionary sig- 

 nificance of the Cestodaria. The merozoic cestode is probably 

 not the descendant of any monozoic cestode now in existence ; 

 but because of the fundamental similarity between the two 

 groups we may ^.ope to find, in the simpler forms meeting the 

 same problems in essentially the same way, some clue to the 

 manner in which occurred the complicated processes by which 

 the hypothetical turbellarian-like ancestor was transformed into 

 the merozoic cestode. Naturally, those cestodes which are second- 

 arily monozoic, Arckigetes and Caryophyllaeus (Lonnberg, 1897) 

 are of little value in this sense. Amphilina (Acipenser) and 

 Gyrocotyle (Chimaera) are primarily monozoic, inhabit hosts 

 of great phylogenetic age, and show fundamental correspondence 

 to the typical cestode structure in all important respects. Gyro- 

 cotyle, the genus with which this paper is concerned, Lonnberg 

 regarded as off the line of cestode evolution, side-tracked by the 

 development of organs of attachment at both ends. The writer 

 hopes to show that only one functional organ of attachment is 

 present, that this is homologous to the cestode scolex, although 



