ON THE DIPLOOHOKDA. 303 



to each, and that the nucleus lies exactly internal to the 

 vacuole in each case. The contrast between the even ring of 

 light vacuoles and the inner ring of dark nuclei makes a beau- 

 tiful and symmetrical section, which can be only poorly indi- 

 cated in the figure. We cannot doubt that the presence of 

 the vacuoles is due to the secretion of some fluid product at 

 the distal extremity of the cell, which is further enclosed in a 

 cell membrane. The close contact of the nucleus with the 

 vacuole is suggestive in this connection. 



In a later stage (figs. 31 and 32) there has arisen another 

 vacuole in each cell, internal to the former. The whole organ 

 is also more elongated. In fig. 31 the vacuoles are mostly 

 alternate to each other, but in certain stages tiiere can be dis- 

 cerned, especially at the area later developed (below v. in 

 fig. 31), that the inner vacuole is first formed exactly internal 

 to the first, and that later on a shifting takes place till the 

 alternate arrangement is arrived at. The latter is evidently 

 the more stable condition, and the assumption of it by these 

 vacuoles tends to indicate that the vacuoles are turgid. In 

 this stage the nuclei are still regularly disposed, the majority 

 situated in an even row internal to the vacuoles, but a few are 

 caught between the latter and are seen squeezed in the chinks 

 here and there. 



These changes may be indicated semi-diagrammatically by 

 figs. 33, 34, and 35. The stage with three rows of vacuoles is not 

 apparently to be obtained, because the vacuoles are later dis- 

 posed rather more irregularly, and the next stage of import- 

 ance is seen in fig. 36. Here the vacuoles have become 

 squeezed together, like fish-eggs allowed to settle slowly in 

 spirit, and some are no longer spherical. The majority of the 

 nuclei are still confined to a small area of protoplasm at the 

 inner end of each cell ; they are not very regularly arranged. 

 Fig. 37 is added to show the condition of this organ as seen 

 in oblique section, within the young Phoronis, soon after 

 the metamorphosis. Its subsequent fate is unknown, but it is 

 not present in the adult. We may note in this final stage 

 that the vacuoles are still further distorted by mutual pressure, 



VOL. 40, PART 2. — NKW SER. Y 



