19] PROTEOCEPHALIDAE — LA RUE 19 



The vestigial fifth sucker in the adult head is represented by a mass 

 of cells or of nuclei and at times a few fibers surrounded by a basement 

 membrane. It is entirely sunken into the tissues of the head and has 

 lost all connection with the exterior. An examination of this structure 

 in an adult head yields no clue to its probable origin, but if heads of 

 plerocercoids of a species which has this structure in the adult be exam- 

 ined it is seen to be a degenerate fifth sucker. In the plerocercoid this 

 degenerating fifth sucker has a sucker cavity communicating with the 

 exterior. It has basement membranes and a full complement of muscles 

 altho some of the muscles may be somewhat displaced and undergoing 

 atrophy. The origin of this structure has been worked out in some 

 detail for Ophiotaenia filaroides and the results are given in the descrip- 

 tion of that species. Its structure is illustrated by drawings which are 

 reproduced here (Figs. 27, 28). The structure of the adult organ was 

 given in the writer's paper on this cestode (La Rue 1909). 



A vestigial fifth sucker which has not reached such a state of 

 degeneracy as is found in many of the species occurs in the head of 

 Crepidohothrium gerrardii. In this species the sucker tho so greatly 

 reduced in size as to be overlooked in an examination of toto prepara- 

 tions still retains its cavity which is yet in connection with the exterior 

 through a small pore. The sucker is surrounded by a basement membrane 

 and it shows some traces of muscles altho the material studied was too 

 poor to make a good histological study. A more complete description 

 of this vestigial sucker is to be found in the description of Crepidohoth- 

 rium gerrardii. Its structure is delineated in figure 42. In his descrip- 

 tion of these vestigial structures found in O. filaroides and in C. gerrar- 

 dii the writer has conclusively shown that the organ under discussion is 

 a vestigial fifth sucker and not a vestigial or rudimentary rostellum. It 

 seems highly probable that similar structures found in other species of 

 the group will prove to be vestigial fifth suckers. 



The writer can see no reason for assuming that this structure is a 

 rudimentary rostellum. So far as known a well developed rostellum 

 does not occur among the Tetraphyllidea but is characteristic of many 

 species of Cyclophyllidea. Unless an apical sucker is a transitional stage 

 in the development (phylogenetie) of a rostellum the writer sees no 

 reason for regarding this vestigial fifth sucker as a rudimentary rostel- 

 lum. The knowledge concerning this structure is incomplete and studies 

 of developmental stages of Proteocephalid species possessing vestigial or 

 functional fifth suckers should be made in comparison with similar de- 

 velopmental stages of species of Calliobothriura and Anthocephalus 

 which according to Lang (1881) and Monticelli (1888) have certain 

 rudimentary structures that seem to be like the oral suckers of trema- 



