LATERAL AND TRANSVERSE EXCRETORY VESSELS. 



299 



From the head the four vessels pass to the neck of the tape-worm, 

 where, with the increasing breadth, they diverge ever further from 

 one another, till they finally acquire a lateral position. 



In many species these lateral stems can be followed throughout 

 the whole chain as four strands, sometimes equally developed, or 

 sometimes with one canal on either side abortive. This unequal 

 growth is seen in very various degrees, and is sometimes much 

 marlied, especially in the larger Tcenice, where one canal much 

 narrower has been observed running for some distance considerably 

 to the inside of the larger (see Fig. 148), but gradually dis- 

 appearing in the broader and thicker joints.^ Instead of the four 

 canals, there is thus but one on either side, but this has a considerable 

 diameter, which has grown in proportion to the joints. This is the 

 state of the case, e.g., in Tcenia saginata and T. solium, while T. elliptica 

 and T. perfoliata, on the other hand, are 

 instances of forms where there are two vessels 

 on either side, in the first case unequally, 

 and in the latter almost equally developed. 



One must not suppose that these vessels run 

 isolated through the body of the tape-worm. 

 As in the head, so in the joints, they are con- 

 nected by a transverse anastomosis, by a 

 circular vessel, when four canals are present, ^ 

 or by a simple vessel, where there are 

 only two. The anastomosis always takes 

 place at the posterior border of the joint 

 (Fig. 147), and has about the^ same diameter 

 as the connected vessels. 



We may further note that the width of 

 the vessels is not always equal, but under- 

 goes many changes, which take place indeed 

 only slowly and gradually, but sometimes 

 are so marked as to result in the dis- 

 appearance of individual vessels. One is 

 sometimes inclined to suppose with v. Siebold (in the case of Tcenia 

 echinococcus), that the walls of the vessels possess a slight but real 



' Monioz connects the abortion of these canals, perhaps not without reason, with the 

 formation and growth of the genitalia {Bull. sci. dep. Nord, p. 225, 1878). He is, how- 

 ever, wrong in regarding the strands of Sonimer, which we have seen to be nerves, as the 

 remains of the aborted vessels. Indeed, he has afterwards recognised their nervous 

 nature (ibid., p. 73, 1879). [According to Pintner's observations, the abortive vessels 

 always lie on the dorsal aspect of the body. — R. L.] 



" Steudener denies the existence of these circular vessels, and asserts that the com- 

 munication between the two sides is always effected^v a simple cross vessel. This is 

 a mistake, as is proved by lM5Sffi?%eaiches dfiPfiflfl^cc/oZioia. 



Fig. 154. — Joint of Tania 

 ccenm-us, with excretory vessels 

 and generative organs. ( x 10.) 



