190 Proceedings of the Royal Physical Society. 



of the stem of the Polian vesicle, and it breaks up into terminal divisions 

 near the level where the stem of the vesicle leaves the interbrachial septum 

 to become free in the body cavity. I was not able to trace any epithelium- 

 lined twigs on to the stem of the Polian vesicle, although it seems to be 

 generally agreed that branches of the radial and the marginal perihaemal 

 canals are continued on to the walls of the sucker feet. 



The foregoing relations are illustrated in Fig. 8, which shows also in 

 section the internal circular sinus and the so-called haemal vessel between 

 them. It will be observed that the ring canal of the hydrocoele is some- 

 what expanded and prolonged downward in the figure. This only occurs in 

 the middle of each interradius, and I am unable to suggest a reason for it. 



Two Tiedemann's bodies are present in each ordinary interradius, arising 

 from the ring canal by separate stalks. For purposes of illustration one of 

 them has been slightly shifted so that its stalk of origin is represented in the 

 figure, which otherwise shows only structures belonging to the mid-interradial 

 line. 



Only one Tiedemann's body is present in the madreporic interradius. It 

 lies to the axial organ side of the stone canal, i.e., the dextral or watch-hand 

 side as viewed from the aboral aspect. 



Although, as was stated above, there is no Polian vesicle in the madreporic 

 interradius, the interbi achial peribsemal branch is here quite definite and 

 ends in the same manner as in the other interradii. Finally, it is to be noted 

 that all the interbrachial perihsemal branches are unaccompanied by haemal 

 tissue. 



A branch of the external perihsemal sinus, similar in position to that just 

 described, has been noted (l,p. 623) by Greef, Hoffmann, Ludwig and Cue'not, 

 but the presence of this branch does not seem to be very generally recognised, 

 and among the authors named there is wide divergence of opinion regarding 

 its mode of termination. According to Greef it communicates with the 

 aboral peribsemal ring, and according to Cuenot with the body cavity, while 

 Ludwig states that it ends by breaking up into twigs and joining the system 

 of definite lacunar channels, which he and others believe to exist between 

 the inner and outer layers of the body wall. The reality of this system of 

 spaces, as described, has been strongly questioned by Macbride and Cuenot. 

 My own series do not show such spaces, but it should be stated that the 

 methods I used, though suitable for ordinary purposes, could hardly have 

 been expected to bring out lymphatic channels elsewhere, ejj., in the tissues 

 of vertebrates. 



