292 



THE ANIMAL PARASITES OF MAN 



especially between the larger branches, and more rarely between the 

 smaller collecting tubes also (fig. 191). 



The so-called " island " formation is another modification, i.e., 

 at any spot a vessel may divide and after a longer or shorter course 

 the two branches reunite, and this may appear in the collecting tubes 

 themselves as well as in their anastomoses. The above-mentioned ring 

 in the frontal commissure of the Tceniidcs is such an island; similar 

 rings also frequently encircle the suckers (fig. 190). In extreme cases 

 (Tricenophorus, Ligula, Dibothriocephaliis, etc.) this island formation 

 extends to all the collecting tubes and their anastomoses. Instead of 

 two or four longitudinal canals only, connected by transverse anasto- 

 moses at the posterior border of the segments, there is an irregular 

 network of vessels, situated in the cortical layer, from which the 



FIG. 189. Young Acanthobothrium 

 corcnatum, v. Ben., with the excretory 

 vessels outlined. Slightly enlarged. (After 

 Pintner.) 



FIG. 190. Scolex of a cysticercoid 

 from Arion sp., with the excretory 

 vessels outlined. (After Pintner.) 



longitudinal branches, having again subdivided, can only be dis- 

 tinguished at intervals, and even then not in their usual number. 



The opening of the longitudinal branches at the posterior end 

 requires more accurate investigation ; it is true that a single terminal 

 bladder is mentioned as being present in many species, but this is 

 also disputed ; when the original end proglottis has been cast off, the 

 longitudinal branches discharge separately. Some species possess the 

 so-called foramina secundaria, which serve as outlets for the collecting 

 tubes ; they are generally at the neck, but may be situated on the 

 segments. 



The contents of the excretory vessels is a clear fluid, the regurgi- 

 tation of which is prevented by the valves present at the points of 

 origin of the transverse anastomoses. The fluid contains in solution 

 a substance similar to guanine and xanthine. 



