Fig. 189.—Young Acantho­both­rium coro­natum, 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.)

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 Tæniidæ is such an island; similar rings also frequently encircle the suckers (fig. 190). In extreme cases (Triænophorus, Ligula, Dibothriocephalus, etc.) this island formation extends to all the collecting tubes and their anastomoses. Instead of two or four longitudinal canals only, connected by transverse anastomoses at the posterior border of the segments, there is an irregular network of vessels, situated in the cortical layer, from which the longitudinal branches, having again subdivided, can only be distinguished 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 regurgitation 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.