ig6 THE ANIMAL PARASITES OF MAN 



which discharges in the middle of the posterior edge of the original 

 terminal proglottis. 



This primitive type (fig. 123) of collective tubes is subject to 

 variation in most cestodes, in the scolex as well as in the seg- 

 ments. Indeed, even the lumen of the four longitudinal tubes 

 does not remain the same, because the dorsal or exterior tubes 

 are more fully developed and become broader, whereas the 

 ventral or inner ones remain small, and in some species quite 

 disappear in the older segments (figs. 119 and 122). Moreover, 

 very frequently connections are established between the right and 

 left longitudinal branches, as in the head, where a " frontal 

 anastomosis " develops, which in the taeniadae usually takes the form 

 of a wreath encircling the rostellurn (fig. 124), and in the seg* 

 ments transverse anastomoses form at each posterior border, 

 especially between the larger branches, and more rarely between 

 the smaller collecting tubes also (fig. 125). 



The so-called " island " formation is another modification, and 

 this may appear in the collecting tubes themselves as well as 

 in their anastomoses ; at any spot a vessel may divide and after 

 a longer or shorter course the two branches reunite. The above- 

 mentioned ring in the frontal commissure of the tajniadse is such 

 an island ; similar rings also frequently encircle the suckers 

 (fig. 124). In extreme cases (Tricenophorus, Ligula, Dibolhrio- 

 cephalus, &c.) 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 secundarja, 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 

 oi departure of the transverse anastomoses. The fluid contains a 

 substance similar to a solution of guanine and[xanthine. 



