24 BULLETIN OF THE 



walls of the stomach and pharynx become separated from each other by 

 a part of the coelomic cavity, as in Plate IV. Figure 36. This process 

 of separation of the alimentary tract proceeds analwards, and finally the 

 rectum is far removed from the oesophagus. 



The anus thus comes to lie farther outside of the anal tentacles. 

 Finally, the ring canal, which is formed progressively farther and farther 

 analwards, follows the fusion of the anal ends of the lophophores, and 

 thus completes the canal behind the oesophagus. (Plate IV. Fig. 36 ; 

 Plate VI. Fig. 53, ccoi. crc.) 



The anal part of the ring canal is doubtless not merely a groove, 

 but a tube ; but the ring canal is not closed at this, and probably not 

 at any stage throughout its entire extent, for in Plate VI. Figure 52, 

 two sections below Figure 53, an opening is shown to exist on each 

 side (at can. crc), putting the cavities of the ring canals and the 

 cffilom into communication with each other. These openings lie at 

 the sides of and slightly above the ganglion (g)i., Fig. 52) ; a position 

 exactly comparable with that of the openings in the ring canal of Phy- 

 lactolaemata, which leads from the coelom into the lophophoric arms on 

 the one hand, and into the circumoral part of the ring canal on the 

 other. 



By a comparison of Figure 41 (Plate IV.) with the sections shown in 

 Figures 60-62, it will be seen that the row of tentacles has undergone 

 a change of form : from being laterally compressed, it has become cir- 

 cular. This change of form has not resulted from an increase in the 

 number of the tentacles, for at the stage of Figure 41 there are six ten- 

 tacles on each side already formed (the sixth not visible), and there 

 are in front of the mouth spaces already reserved for the two additional 

 tentacles. There are also, probably, two nascent tentacles at the anus, 

 although these are little developed, making a total of 16. In Figure 61 

 there are only 15 tentacles ; moreover, the actual diameter of the ten- 

 tacular corona in the sagittal plane is less than at the earlier stage 

 of Figure 41. This change of form is perfectly normal, all young 

 polypides having tentacles arranged in two parallel rows, and adult 

 polypides having a circular lophophore. 



These changes in the form of the tentacular corona are correlated 

 with important changes in the direction of the axes of other organs. 

 These changes may be understood by comparison of Figui-es 25 and 36, 

 together with the assistance of Figures 7-9, all of which are oriented 

 in the same manner. In Figure 25 the points fixed by the cardiac valve 

 (vlv. or.) and anus (a)i.) lie in a line which is approximately parallel to 



