CESTODES 189 



likewise appear in varying numbers and different positions, they 

 may be on the suctorial organs or above them on the apex 

 of the scolex ; for instance, in many of the Taeniadae 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) ROSTRUM. One family of the cestodes, the 

 Rhynchobothriidia carries four rostra, 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 part connecting it with 

 the proglottides 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 

 proglottides which is transformed into the so-called PSEUDOSCOLEX. 



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

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

 oldest proglottis is furthest from the scolex, ' and the youngest 

 nearest the neck. 



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 

 longitudinally or perpendicularly to that of the entire chain ; 

 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, even 

 starting at the neck, are very flat ; in rare cases their transverse 

 diameter is equal to their dorso-ventral diameter. As a rule the 

 segments, singly, or several united together, detach themselves from 

 the posterior end, in many cases only after complete maturity is 

 attained, and in others much earlier ; they then continue to live 

 near their parent colony in the same intestine and continue their 

 development. Even when evacuated from the intestine the pro- 

 glottides under favourable circumstances can continue to live and 

 creep about until they perish, sooner or later. 



The first proglottis formed, and which in a complete tape- 



