300 THE ANATOMY OF CESTODES. 



power of contracting, were it not, however, possible that the phenomena 

 observed were due to external pressure or to internal obstruction. At 

 any rate, the vessels even the widest of them are without circular 

 muscles. Their walls consist of a clear, structureless cuticular mem- 

 brane, which is of varying thickness, according to the width of the 

 lumen, but is otherwise without peculiarity. A coating of cells has 

 never been observed, even on the outer surface of the vessels, which is 

 on all sides in direct contact with the connective-tissue substance in 

 which they are embedded. 



Here and there one can observe individual fibres attaching them- 

 selves to the walls of the vessels. The connection is effected by means 

 of a small wing-like or conical terminal piece, which is somewhat like 

 the so-called " terminal triangle " of the motor nerve fibres. I can 

 hardly doubt that these fibres have a muscular character, and that they 

 are able to affect the width of the vessels, especially since they lie at 

 right angles to the walls. If we suppose, what is indeed warranted 

 by the optical characters of the walls, that the canal system of the 

 Cestodes is in a state of elastic tension, even when the vessels are 

 moderately full, then the presence of these special dilators is very 

 natural, and well suited to increase or to oppose the action of obstruc- 

 tion or pressure, as the case may be. The local limitation of these 

 actions is readily intelligible, since the contents of the vessels can 

 only move backwards, in spite of the continuity of the whole system. 

 It has never been found possible to fill the longitudinal stems by in- 

 jecting substances from behind forwards, while there is no difficulty in 

 driving the fluid from the head downwards through a long stretch of 

 joints. This is explained by the presence of a valvular arrange- 

 ment, which is formed (according to Platner and Sommer) by two 

 opposite folds or duplicatures of the walls, which project into the 

 lumen at a point above the transverse anastomoses. 



This arrangement would of course result in a constant accumula- 

 tion of excreted matter in the posterior joints, were there not some 

 means for its removal. And so there is, for at the end of the last 

 joint the vascular apparatus opens to the exterior. 



This is generally effected by means of the transverse anastomoses. 

 In the posterior joints which are sharply divided from one another, the 

 transverse branches shorten according to the depth of the constriction, 

 and finally, when the joints separate and the vessels rupture, the line of 

 anastomosis becomes a cross cleft. This has no great length, and, 

 curving forward, assumes a sort of bladder-like form, while the line of 

 rupture is at the same time drawn together. It is this bladder which 

 receives the longitudinal vessels and conducts their contents to the 

 exterior, for which function it is specially fitted by the presence of the 



