122 THE CESTOIDEA 



is more or less modified by fusion of the phyllidia. In Dinobothrium 

 the phyllidia are arranged in two pairs, the members of a pair being 

 close together. In Diplobothrium, Zygdbothrium, and Platybothrium 

 the paired members are actually united, so that the head appears to 

 possess only two instead of four phyllidia ; but evidence of fusion 

 is provided by the existence of a slight ridge traversing each 

 apparent phyllidium. A step further, and the four phyllidia may 

 fuse to form a single structure, as in Gamobothridae, giving rise to a 

 plate-like or globular termination to the head (Fig. XVII. 3, 4). 



In Thysanocephalum (see 25) the scolex is hidden by a great 

 swelling of the neck, which becomes folded and fringed so as to 

 give rise to a structure recalling the mobile organ of Gyrocotyle, 

 much exaggerated (Fig. XVII. 2). Here the " neck " is replacing, 

 functionally, the scolex, which is greatly reduced, and suggests that 

 possibly in " Phyllobothrium laduca" v. Ben., something of the same 

 sort has occurred. 



The " rostellum " or apical region beyond the attachment of the 

 phyllidia may or may not be present ; it undergoes great modifica- 

 tions in some cases, and possibly in the Gamobothridae this organ 

 shares in the formation of the very peculiar "head" (see 25). It 

 may carry a terminal sucker (Echeneibothrium, Fig. XVI. 4), as in 

 some of the Tetracotylea. 



The four phyllidia or other organs of the scolex in the Tetra- 

 bothridiata are normally arranged at equal distances around the 

 apex of the scolex, in such a way that, as the nervous and excretory 

 systems show, two correspond to the dorsal and two to the ventral 

 surfaces of the proglottids ; in other words, each bothrium of 

 Bothriocephalus is represented, topographically at least, by a pair of 

 organs in the Tetrabothridiata. It is not, however, probable that 

 the latter organs are descended from such a Dibothridiate form by 

 a process of duplication. We have no evidence from embryology 

 that this process takes place ; on the contrary, we have evidence 

 that the reverse process obtains in some of the Tetrabothridiata. 

 Nevertheless, the fact that, in the Dibothridiata, the generative 

 organs resemble those of the Cestoidea Monozoa and Heterocotylea, 

 forbids us taking the view that each of the two "bothria" in 

 this branch has arisen by fusion of a pair of ancestral organs. 

 We have, therefore, to fall back on the assumption that both 

 groups have been derived, along divergent lines, from a common 

 ancestor in which the organs of adhesion were not yet developed, 

 or possibly were in some such condition as that of Caryopliyllaeus. 



The generative organs retain in some respects the arrange- 

 ment found in the Dibothridiata, viz. the vitellarium is marginal 

 and follicular, extending round the entire proglottid ; the vagina 

 is usually anterior to the penis ; the uterus is more or less tubular 

 and undulating, but has lost its pore. The genital pore is 



