354 University of California Publications in Zoology. C V L - 6 



A. INTRODUCTION. 



The Cestodaria (Monticelli, 1892) or monozoic cestodes 

 (Lang, 1891), including Amphilina, (Rud.), Archigetes (Ratz) 

 Caryophyllaeus, (Rud.) Gyrocotyle, (Diesing) and Wageneria 

 (Wag.) are of peculiar interest to students of the platyhelminths. 

 They occupy a position intermediate between the merozoic ces- 

 todes and the trematodes, being allied to the latter by the general 

 arrangement of the reproductive organs and by their funda- 

 mentally unitary character, and distinguished from them by the 

 absence of a digestive tract. This last characteristic allies them 

 with the merozoic cestodes, from which they are distinguished 

 on the other hand by their unsegmented condition and by the 

 accompanying absence of segmental repetition of the genital 

 organs, as stated by Monticelli (1892). The group is difficult 

 to consider as a whole because of the remarkable diversity shown 

 in its four well-established genera, both as to structure, life-his- 

 tory and host. The characters mentioned above practically 

 exhaust those common to the group, and each genus possesses 

 characters of such rank as fittingly to receive ordinal distinction 

 in classification. Two of the genera, Archigetes and Wageneria, 

 have but one species each. Two each have been assigned to 

 Caryophyllaeus and Amphilina, and at least four are now recog- 

 nized in the genus Gyrocotyle. The grounds on which specific 

 distinctions have been made are far from satisfactory. These 

 considerations indicate that the group is not only phylogenetic- 

 ally intermediate but composed of very old, long-established 

 forms, on which the forces that make for speciation have long 

 since ceased to act. The Cestodaria are either remnants of a 

 once large and differentiated group, or they are members of a 

 class never differentiated to any considerable extent. They are 

 presumably of greater age than the cestodes, their hosts being 

 invertebrates or primitive vertebrates. Cestodes are phylogen- 

 etically the youngest of the platyhelminths, since the host in 

 the sexual period is always a vertebrate. They have been greatly 

 and variously modified by their parasitic habit, and they present 

 problems of structure and development of extreme difficulty. It 

 seems probable that much help towards the solution of these can 



