CESTODA. 659 



into one of the main longitudinal canals, e. g. Scolex Trygonis pastinacae. 

 These canals take a longitudinal course. They number four two on each 

 side, one dorsal, the other ventral in the Taeniadae, Tetrarhynchidae, and 

 Tetraphyllidae, As to the first-named, the dorsal canals have a tendency 

 to atrophy, and may even disappear, and this is especially the case with the 

 large tapeworms that infest man *. The four canals are connected in the 

 head by a simple or plexiform ring ; at the posterior margin of each joint 

 by a circular vessel, or when the ventral main canals alone persist by a 

 cross vessel. In some instances the anastomoses are more complicated 

 (Riehm). A valve, formed by two opposing folds of the walls of the canal, 

 guards the aperture at either end of the cross vessel. The Pseudophyllidea, 

 however, and Caryophyllaeus possess as a rule a larger number of main 

 canals, 10-24, or even more, according to the genus or species ; and Fraipont 

 has drawn a distinction between ascending and descending canals. The 

 former have a smaller calibre, contents rather granular, and are non- 

 contractile ; the latter are larger, with clear contents, and distinctly con- 

 tractile. The two sets of vessels are connected by a network of vessels 

 intermediate in size between them and the canalicules (supra). And it 

 must be carefully noted that the different systems of vessels, i. e. main 

 canals, intermediate network, and canalicules, invariably retain their own 

 proper calibre and never graduate the one into the other. Their walls are 

 formed by a delicate membrane. Those of the main canals are surrounded, 

 according to Pintner, by an epithelium, the cells of which contain yellow 

 droplets. In Solenophorus similar cells occur in the matrix in the neigh- 

 bourhood of the canals, but not in contact with their walls (Griesbach) ; 

 whilst in Taenia lineata flattened granular epithelioid cells immediately in- 

 vest them (Hamann). The point is one requiring further investigation. 

 The excretory system opens externally by a posteriorly placed pulsatile 

 vesicle, with which the main canals are connected, e. g. in Caryophyllaeus, 

 some species of Bothriocephalus, &c. ; and when joints have been detached 

 either by a shortened cross vessel, as in Taeniadae according to Leuckart, 

 or by separate openings, one for each of the main canals, apparently the 

 more usual mode. The pulsatile vesicle is always present, according to 

 Pintner, at the end of the body of the scolex. The main canals have also 

 been observed to open externally at the margins of the body by short 

 tubes ; on the head, e. g. in Tetrarhynchus ; on the neck as well, e. g. Scolex 

 Trygonis pastinacae, some Taeniae and also on the joints, e.g. Bothriocephalus 

 punctatus, Triaenophorus, Taenia osculata. Two such openings in every 



1 According to Moniez ('Les Cestodes,' p. 185 et seqq.) the ventral vessel in Taeniae belonging 

 to the type of T. serrata becomes lacnnar ; the dorsal appears to vary much. The cross anastomosis 

 mentioned later on connects only the ventral vessels. But in the abortive joints of T. crassicollis and 

 in Tetrarhynchus claviger both vessels have their cross' anastomoses, which may fuse medianly ; and 

 in the first-named there is evidence for a connection between the ventral and dorsal vessels, establishing 

 thus a circular communication. 



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