274 



CE8TODBS. 



FIG. 135. 



looked for between the suckers (Bremser, Mehlis), at a point on the 

 top of the head, where the outer coverings are often drawn together 

 into a depression, and where, in some species, a minute but distinct 

 sucker (the frontal sucker) is formed. We know, however, that the 

 Cestodes are wholly destitute of both mouth and alimentary canal. 

 The observation of two repeatedly described longitudinal channels 

 suggested the existence of an alimentary canal, and therefore lent 

 some probability to the above belief in the presence of a mouth, but 

 we shall afterwards see that they are only the main stems of the so- 

 called " excretory vascular " system. 



The physiological significance of the head referred to above is 

 evidently important, since the suspended sexual animals are destitute 

 of hooks. The nurse undertakes the attachment of the colony, and 

 thus has its specific share in the general economy, as is usual in 

 cases of polymorphism. 1 



The association of sexual members (the so-'called " Proglottides ") is 



not, however, a lasting one. Sooner or 

 later the terminal joints become loose, 

 sometimes singly, sometimes in groups, 

 and after remaining for a while in in- 

 dependent freedom beside their still 

 attached fellows, are carried along with 

 the faeces from their intestinal home. 

 As a rule, this separation takes place 

 only after the perfect development of 

 the discharged sexual members, and 

 often, as in Tccnia, after even their 

 embryos are mature; 2 but there are 

 cases where the joints are liberated 

 much earlier. Thus, thanks to van 

 Beneden, we know of certain forms 

 from the alimentary canal of the pre- 

 datory rays and sharks, whose proglot- 

 tides only reach maturity after they 

 have become free, and which, in their 

 isolated state, grow to a size almost as 

 great as that of the whole tape- worm, 

 from which they originated. With 

 such cases before us, every doubt as 

 to the independent nature of the 

 individual joints must vanish. 



1 See Leuckart, " Ueber den Polymorphismus der Individuen oder die Erscheinungen 

 der Arbeitstheilung in der organischen Natur:" Giessen, 1852. 



2 Hence the name " Ovaria amlmlnntia," sometimes applied by Pallas to the sexual 

 animals of Ttenia. 



FIG. 136. 



FIGS. 135 and 136. Strobila and 

 Proglottis of Echineibothrium mini- 

 mum (after van Beneden). 



