298 THE ANATOMY OF CESTODES. 



layer of cells which Schneider described as lying on the dorsal and 

 ventral surface. 



The Alimentary System. We have already noted how the struc- 

 ture of the Cestodes is simplified by the absence of these organs. Nor 

 is it the alimentary canal alone which is wanting, but blood-vessels 

 and blood are also sought for in vain. The transference of the nutritive 

 fluid is wholly effected by osmosis as in the other bloodless animals, 

 which include a great many of the parenchymatous worms. 



Attempts have, indeed, been made to credit the Cestodes with 

 these structures. It used to be very common to describe an alimen- 

 tary canal, and even a two-lobed one, as in the majority of the Trema- 

 todes. Observers spoke of two vascular canals running laterally 

 throughout the whole chain, and opening to the exterior, sometimes 

 on the top of the head, sometimes by the suckers. Longitudinal 

 canals the Cestodes certainly do possess, which run uninterruptedly 

 from head to terminal joint, and which are sometimes straight, or 

 sometimes bent into zigzag folds, according to the state of contrac- 

 tion exhibited by the animal. But these canals are not alimen- 

 tary ; they have no mouth opening to the exterior ; they contain no 

 chyme, but only a clear watery fluid, without granules, and at most 

 yielding a finely granular precipitate on treatment with certain re- 

 agents (absolute alcohol). This substance is occasionally expelled in 

 the form of little pillars of varying length. The chemical analysis of 

 these masses yields, according to Sommer, sub- 

 stances which are related to xanthin and guaniii. 



The Excretory System. From what has just 

 been said, we may take it as proven that the 

 longitudinal canals of the Cestodes have an 

 excretory function, as has indeed been for long 

 very generally assumed from the analogy of the 

 Trematodes. x 



The arrangement of the vessels is not the 

 same in all the Cestodes, nor are they typically 

 two, but rather four in number, particularly in 

 , . forms with four suckers. Four are generally to be 



FIG. 153. Head of > . . J 



Twnia term/a, with its seen in the head, corresponding in position to the 

 excretory vessels ( x 24). suc k erS) an( j urnting under the rostellum, when 

 there is one, by means of a simple or plexifonn circular vessel. 



1 The term *' water vessels," introduced by v. Siebold to denote these canals, is by 

 no means happy. In the first place, the contents do not consist of water, and further, 

 the name is associated with v. Siebold's erroneous idea that the "water vessels" were 

 respiratory (" Vergl. Anat.," pp. 43 and 137, 1848). Further, the term has been used to 

 denote the most diverse structures. See my remarks in Bergmann and Leuckart, " Vergl. 

 Anat. u. Physiol.," p. 284, 1851. 



