ANATOMY OF THE CESTODA 285 



intestinal wall, and therefore carries at its end. various organs which 

 assist in this function, and which are as follows : (i) SUCTORIAL 

 ORGANS, i.e., the four suckers (acetabula), which are placed crosswise 

 at the circumference of the thickened end of the scolex ; further, the 

 double or quadruple groove-like suckers (bothridia), which are 

 diversely shaped in the various genera and families. 1 (2) FIXATION 

 ORGANS (hooklets) 2 that likewise occur in varying numbers and 

 different positions; they may be in the suckers, or outside them on the 

 apex of the scolex ; for instance, in many of the Tceniidce they appear 

 in a circle around a single protractile organ, the rostellum, or the latter 

 may be rudimentary, and is then replaced by a terminal sucker. 



(3) PROBOSCIS. One family of the Cestodes, the Rhynchobothriidce, 

 carries four proboscides, moved by their own muscular apparatus, 

 on the scolex, and they are beset with the most diverse hooks. 



(4) TENTACLE-LIKE formations are only known in one genus 

 (Polypocephalus). 



The thickened part of the scolex that carries the suckers is usually 

 called the head ; the following flat (unsegmented) part connecting it 

 with the proglottids is called the neck, and is sometimes quite small. 

 In a few cases the entire scolex (or head) disappears, and its function is 

 then undertaken by the contiguous portion of the chain of proglottids, 

 which is transformed into a variously shaped PSEUDO-SCOLEX. 



The proglottids are joined to the scolex in a longitudinal row, 

 and are arranged according to age in such a manner that the oldest 

 proglottis is farthest from the scolex, and the youngest nearest to it. 



The number of segments varies, according to the species, from 

 only a few to several thousands ; they are either quadrangular or 

 rectangular ; in the latter case their longitudinal axis falls either 

 longitudinal or transverse to that of the entire chain, according as 

 the segments are longer than broad or broader than long. When the 

 number of segments is very large, the youngest ones are, as a rule, 

 transversely oblong, the middle ones are squarish, and the mature 

 ones longitudinally oblong. The posterior border of the segments, 

 as a rule, carries a longitudinal groove for the reception of the shorter 

 anterior border of the following proglottis. The two lateral borders 

 of the segment are rectilinear, but converge more or less towards the 

 front, or they are bent outwards. In most of the Cestodes the 

 segments, just as the neck, are very flat ; in rare cases their 



1 They may remain simple, and are then not separated from the remaining muscles 

 of the scolex ; or they project as roundish or elongated structures over the scolex, hollow on 

 their free surface, and often divided into numerous areas by muscular transverse ribs. They 

 may also carry accessory suckers on their surface. 



2 The various parts of a hooklet are thus named from the point backwards : (i) blade or 

 prong, (2) guard or ventral or posterior root, (3) handle or dorsal or anterior root. 



