18 ILLINOIS BIOLOGICAL MONOGRAPHS [IS 



rostellum. Such a supposition, however, is incorrect, for in its devel- 

 opment it is a fifth sucker. A more complete discussion of this rudi- 

 mentary or vestigial fifth sucker is given under the proper caption 

 (vide infra). 



All species are provided with four suckers and some with an apical 

 fifth sucker in addition. The latter may be either functional or vesti- 

 gial. The four suckers are sessile, prominent, or inconspicuous cup- 

 shaped organs which are usually situated on the broadest part of the 

 head or just anterior to the broadest zone. In Corallobothrium the 

 suckers are situated on the flattened anterior face of the head. The 

 suckers do not possess accessory areola nor are they provided with hooks 

 in their cavities or on their margins. Acanthotaenia may have cuticular 

 spines within the sucker cavity. Suckers are usually rounded or oval 

 in outline and may have deep cavities. The margin is usually entire 

 but is at times interrupted. Some peculiar cases occur : Proteocephalus 

 cyclops has suckers pointed at the posterior end. The suckers of 

 P. singularis (Fig. 25) are flat, thin and weakly muscled and have a 

 peculiar upturned point at the anterior margin. The deep grooves be- 

 tween the suckers of this species cause these organs to stand out promi- 

 nently. Crcpidobothriuni gerrardii (Figs. 12, 13, 33, 34, 123, and 124) 

 likewise has peculiar suckers in that each has an interrupted lower 

 margin which forms a point re-entrant into the sucker cavity. Deep 

 grooves between the suckers give the head a lobed appearance and cause 

 the suckers to be prominent. Proteocephalus osculatus (Fig. 162) has 

 a fifth sucker which is said to be covered with minute spinelets. The 

 suckers of some of the Acanthotaenia are covered with minute spinelets. 

 It is deemed unnecessary to enter into a discussion of the histology of the 

 suckers of this group. Benedict (1900), Kraemer (1892), and La Rue 

 (1909) have discussed this subject in detail. 



A functional fifth sucker situated at the apex of the head is present 

 in many species of Proteocephalus but is not known among the other 

 genera of the family. Other species of Proteocephalus, Ophiotaenia, 

 Crepidobothrium, and Acanthotaenia are known to possess a structure 

 which the writer (1909) called an end-organ and which Johnston (1909 

 et seq.) has called an apical muscle-plug. Other species of Proteocepha- 

 lus and all known species of Corallobothrium and Choanoscolex do not 

 possess this structure or the functional fifth sucker while some species 

 of Proteocephalus and Ophiotaenia have not been investigated for it. 

 The writer has now determined that this organ is a vestigial fifth sucker. 

 The fifth sucker when functional is usually smaller than the others and 

 it possesses all the histological structures of other Proteocephalid suckers. 

 The basement membrane and the muscles have the same relations as in 

 other suckers. 



