TAPE-W0EM8 OF THE SUD-FAMILY AVITELLININ.E. 361 



Having ascended thus far, it again turns and descends 

 parallel to its ascent ; on reaching the base of the sucker it 

 re-crosses the base of the scolex exterior to the dorsal canal, 

 but interior to its first horizontal stretch. Arrived at the 

 median internal side of the base of the right ventral sucker, 

 it ascends the median ventral side of the sucker to the height 

 of its month, then turns back parallel to the ascending 

 portion of the canal. Passing around the base of the right 

 ventral sucker internally, it ascends the outer angle of the 

 two right suckers, sending off two blind branches towards 

 the outer surfaces of the suckers at the level of the nerve 

 commissure. The main canal continues its course parallel to 

 the long axis of the worm to near the apex of the scolex; 

 then it bends over, descends the inner angle of the rio-ht 

 suckers until the nerve commissure is reached, when it bends 

 over again, proceeding past the nerve commissure ventral to 

 and parallel to the dorsal canal. Arrived at the summit of the 

 scolex, it unites with the dorsal canal and with the connect- 

 ing commissure in the same way as the left canal does. The 

 right dorsal canal is straight thronghout, agreeing in this 

 respect with the left dorsal canal. 



The " blind " branches given off by the ventral canals I 

 have not been able to follow further than stated ; of course, 

 they may be much longer than their observed length. Bearing 

 in mind the difficulty of tracing the transverse commissures in 

 the proglottids of Avitellina centripunctata (Rivolta) for 

 any length (I had not seen them at all in 1909), I certainly 

 hesitate to say that a ring-canal is not formed ; these branches 

 may represent portions of a ring-canal, which, in that case, 

 would be a ti*ansverse canal in the scolex homologous to the 

 transverse commissures in the proglottids. 



It is probable that the various loops change their course 

 considerably according to the state of contraction of the 

 scolex. Stilesia hepatica, Wolffhiigel, also invariabl}^ 

 shows a similar dorsal bend of the ventral canal at the base 

 of the scolex ; the entire course, however, I have not been 

 able to study. 



