THE CESTOIDEA 127 



four phyllidia set in dorsal and ventral pairs may be distinct, 

 terminal, and almost cup-shaped (Synde&mobothrwm), but more gener- 

 ally the pair of organs are adnate (Tetrarhynchus), or even more or 

 less closely united (Otobothrium), but there is frequently a ridge, or 

 other sign of fusion. However, in some cases this fusion is so 

 complete externally that there appears to be only two phyllidia, as 

 in Diphyllidea, but even then, the four structures are distinctly 

 recognisable in section by the distribution of the nerves and 

 excretory organs (lihyncJwbothnum). 



The characteristic structures of the order, however, are the four 

 " proboscides " or trypanorhynchi (Fig. XIX. 4). 



Each proboscis consists of three parts : (1) a longer or shorter 

 hollow tentacle, capable of eversion and introversion, and armed 

 with numerous booklets arranged in a definite way. This tentacle 

 can be withdrawn into (2) the tubular " membranous sheath," which 

 starts from the apex of 'the head where it opens and passes back- 

 wards iuto the head stalk, taking a straight or undulating course ; 

 it terminates in (3) the "muscular bulk" or cylinder, the walls of 

 which are formed of ten or more concentric sheaths of muscle 

 fibres, which are transversely striated. The w T hole apparatus is 

 filled with a fluid, by the compression of which in the " bulb " the 

 tentacle is evaginated. Introversion is effected by a "retractor 

 muscle " attached at one end to the wall of the bulb, at the other 

 to the tip of the tentacle. The apparatus presents certain interest- 

 ing histological features, for instance, the sheath is lined by a 

 distinct epithelium. Each individual proboscis resembles in some 

 degree the single proboscis of Acanthocephala, in a less degree that 

 of Nemertines. It has been usual to homologise these " proboscides " 

 of Tetmrhynclius with the retractile rostellum of certain Taeniidae, 

 but apart from the difference in number, there are differences in 

 structure which militate against this view. It appears more 

 probable (Pintner) that each proboscis has been developed by the 

 deepening and modification of an " accessory sucker " of some 

 Tetraphyllidean, as its relation to the bothridia, and its mode of 

 development, closely agree with these structures. Functionally, 

 too, it is a perfection of the armature plus the accessory sucker of 

 three forms; whilst there is no doubt that the " phyllidia'' of the 

 two orders are identical. As will be shown below, the " acetabula " 

 of Tetracotylea are derived also from the "accessory sucker," so 

 that the proboscis of Tetmrhynchus is homologous with the sucker 

 of Taenia. 



The excretory (Fig. VII. 3) and nervous systems, which 

 present nothing of any systematic value, have been worked out 

 by Lang, Pintner, etc. As to the generative organs, they are 

 constructed on the plan of the Tetraphyllidea, with marginal vitel- 

 laria, etc. 



