THE CESTOIDEA 



121 



as Tristomum or Aspidogaster. This loculation no doubt aids in 

 attachment, (b) In the sub-family Phyliacanthinae, (Fig. XVII. 1) 

 booklets may be added which convert the originally locomotor 

 organ into a more useful adhesive organ, (c) But both in the 

 simple and in the armed phyllidia an " accessory sucker " may be 

 present, either somewhere within the area of the cup (Tetrabothrium), 

 or frequently above it (PhyUobothrium) on the scolex; this accessory 



Fio. XVII. Scolex of various Tetraphyllid 



l.CalliobothriumJUicolle, Zsch., out of Torpedo ocellata (after Zschokke). The phyllidia are 

 here short ; each bothridium is trilocular, and is armed, distally, with two bifurcate hooks. 

 Above each bothridium (e) is an accessory sucker (d). 



2. Thysanocephcdum, crispum, Lint., out of Gctieocerdo tigrinus (after Linton). The small 

 scolex (/) is, during life, partially concealed by the " pseudoscolex ' (g), which is formed by the 

 development of much-folded ridges from the neck. Each bothridium (/) is bilocular, and armed 

 with two simple spines. 



3. tecanicephalum peltatum, Lint. , out of Trygon centrum (after Linton). Viewed from above. 



4. The same from the side. The scolex is here flattened so as to form two horizontal plates, 

 the margins of which (a, b) are membranous and folded, and separated by a slight furrow. The 

 lower plate carries four accessory suckers (c), two of which are indicated by the dark spots in 

 3, and one in 4, in which, however, the index line stops short. 



5.Discocephalum pikatum, Lint., out of Carcharius obscurus (after Linton). The scolex 

 is a flattened, mushroom-shaped disc ; below it is a swollen neck (g), the surface of which is 

 irregular ; it is separated by a groove from the scolex. 



sucker differs in structure from the " bothridium " ; it is a pit in 

 the surface, with strong muscular walls, which are delimited from 

 the parenchyma ; in fact, these " suckers " have the structure of 

 the "acetabula" of the Tetracotylea, with which Pintner (1896) 

 homologises them, regarding the " bothridium " as something not 

 represented in the majority of the Tetracotylea. 



But in addition to forms, in which the four organs are distinctly 

 separate, there are other forms in which the structure of the head 



